Disable tests trying to look up remote servers.

diff --git a/test/test_credentials.py b/test/test_credentials.py
--- a/test/test_credentials.py
+++ b/test/test_credentials.py
@@ -68,6 +68,7 @@ class CredentialCreateTest(utils.NoRepoTestCase):
         self.assertEqual((username, None, None, None), cred.credential_tuple)
 
 
+@unittest.skipIf(True, "network tests are not supported in Guix")
 class CredentialCallback(utils.RepoTestCase):
     def test_callback(self):
         class MyCallbacks(pygit2.RemoteCallbacks):
@@ -92,6 +93,7 @@ class CredentialCallback(utils.RepoTestCase):
         remote = self.repo.create_remote("github", url)
         self.assertRaises(TypeError, lambda: remote.fetch(callbacks=MyCallbacks()))
 
+@unittest.skipIf(True, "network tests are not supported in Guix")
 class CallableCredentialTest(utils.RepoTestCase):
 
     def test_user_pass(self):
diff --git a/test/test_repository.py b/test/test_repository.py
--- a/test/test_repository.py
+++ b/test/test_repository.py
@@ -573,6 +573,7 @@ class CloneRepositoryTest(utils.NoRepoTestCase):
         self.assertTrue('refs/remotes/custom_remote/master' in repo.listall_references())
         self.assertIsNotNone(repo.remotes["custom_remote"])
 
+    @unittest.skipIf(True, "network tests are not supported in Guix")
     def test_clone_with_credentials(self):
         repo = clone_repository(
             "https://bitbucket.org/libgit2/testgitrepository.git",
diff --git a/test/test_submodule.py b/test/test_submodule.py
--- a/test/test_submodule.py
+++ b/test/test_submodule.py
@@ -42,6 +42,7 @@ SUBM_PATH = 'submodule'
 SUBM_URL = 'https://github.com/libgit2/pygit2'
 SUBM_HEAD_SHA = '819cbff552e46ac4b8d10925cc422a30aa04e78e'
 
+@unittest.skipIf(True, "network tests are not supported in Guix")
 class SubmoduleTest(utils.SubmoduleRepoTestCase):
 
     def test_lookup_submodule(self):
value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=1d4f0019f926ce3429c9958330747df996aa709a'>root</a>/<a href='/guix/log/gnu?id=1d4f0019f926ce3429c9958330747df996aa709a'>gnu</a>/<a href='/guix/log/gnu/tests?id=1d4f0019f926ce3429c9958330747df996aa709a'>tests</a>/<a href='/guix/log/gnu/tests/nfs.scm?id=1d4f0019f926ce3429c9958330747df996aa709a'>nfs.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/tests/nfs.scm?id=1d4f0019f926ce3429c9958330747df996aa709a&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2018-10-18 01:12:22 +0200'>2018-10-18</span></td><td><a href='/guix/commit/gnu/tests/nfs.scm?id=39d7fdce453b0ca23ecbed72048647debbaa58a6'>services: dhcp-client: Deprecate 'dhcp-client-service' procedure.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/networking.scm (dhcp-client-service-type): Add default
value.
* gnu/system/examples/bare-bones.tmpl: Use (service
dhcp-client-service-type) instead of (dhcp-client-service).
* gnu/system/examples/beaglebone-black.tmpl: Likewise.
* gnu/tests/base.scm (%avahi-os): Likewise.
* gnu/tests/databases.scm (%memcached-os): Likewise.
(%mongodb-os): Likewise.
* gnu/tests/dict.scm (%dicod-os): Likewise.
* gnu/tests/mail.scm (%opensmtpd-os): Likewise.
(%exim-os): Likewise.
(%dovecot-os): Likewise.
* gnu/tests/messaging.scm (run-xmpp-test): Likewise.
(run-bitlbee-test): Likewise.
* gnu/tests/monitoring.scm (%prometheus-node-exporter-os): Likewise.
* gnu/tests/networking.scm (%inetd-os): Likewise.
(run-iptables-test): Likewise.
* gnu/tests/nfs.scm (%base-os): Likewise.
* gnu/tests/rsync.scm (%rsync-os): Likewise.
* gnu/tests/ssh.scm (run-ssh-test): Likewise.
* gnu/tests/version-control.scm (%cgit-os): Likewise.
(%git-http-os): Likewise.
(%gitolite-os): Likewise.
* gnu/tests/virtualization.scm (%libvirt-os): Likewise.
* gnu/tests/web.scm (%httpd-os): Likewise.
(%nginx-os): Likewise.
(%varnish-os): Likewise.
(%php-fpm-os): Likewise.
(%hpcguix-web-os): Likewise.
(%tailon-os): Likewise.
* tests/guix-system.sh: Likewise.
* doc/guix.texi (Networking Services): Document
'dhcp-client-service-type' and remove 'dhcp-client-service'.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-06-13 09:11:45 +0200'>2018-06-13</span></td><td><a href='/guix/commit/gnu/tests/nfs.scm?id=c24b15472baaa7ec67d48cbf296bbe5ab727b78f'>tests: Honor the return value of 'start-service'.</a><span class='msg-avail'>...<span class='msg-tooltip'>Since commit dc7b3e56337ee9d8dcd8fe7d5cab71ef536d024f, 'start-service' returns
the Shepherd's representation of the service as a sexp, and '#f' if the
service fails to start.  Also, it doesn't throw an exception when the service
fails to start, so relying on an exception instead of relying on its return
value is a false positive.

* gnu/tests/base.scm (run-nss-mdns-test): Split tests so to check the return
value of 'start-service'.
(run-mcron-test, run-nss-mdns-test): Use 'test-assert' with the return value
of 'start-service'.
* gnu/tests/admin.scm (run-tailon-test): Idem.
* gnu/tests/dict.scm (run-dicod-test): Idem.
* gnu/tests/mail.scm (run-opensmtpd-test, run-exim-test, run-dovecot-test):
Idem.
* gnu/tests/messaging.scm (run-xmpp-test, run-bitlbee-test): Idem.
* gnu/tests/nfs.scm (run-nfs-test): Idem.
* gnu/tests/rsync.scm (run-rsync-test): Idem.
* gnu/tests/ssh.scm (run-ssh-test): Idem.
* gnu/tests/version-control.scm (run-cgit-test, run-git-http-test): Idem.
* gnu/tests/web.scm (run-php-fpm-test): Idem.
</span></span></td><td>Clément Lassieur</td></tr>