Fix compatibility with procps 4.

Negative UIDs are no longer allowed.  Use a very high one instead.

Taken from upstream:

  https://github.com/ganeti/ganeti/commit/9cd67e6a81c66ed326d68ea8c3241d14eea6550b

diff --git a/test/py/ganeti.uidpool_unittest.py b/test/py/ganeti.uidpool_unittest.py
index b2f5bc5cf2..2d9227cbf5 100755
--- a/test/py/ganeti.uidpool_unittest.py
+++ b/test/py/ganeti.uidpool_unittest.py
@@ -106,23 +106,24 @@ def testRequestUnusedUid(self):
 
     # Check with a single, known unused user-id
     #
-    # We use "-1" here, which is not a valid user-id, so it's
-    # guaranteed that it's unused.
-    uid = uidpool.RequestUnusedUid(set([-1]))
-    self.assertEqualValues(uid.GetUid(), -1)
+    # We use 2^30+42 here, which is a valid UID, but unlikely to be used on
+    # most systems (even as a subuid).
+    free_uid = 2**30 + 42
+    uid = uidpool.RequestUnusedUid(set([free_uid]))
+    self.assertEqualValues(uid.GetUid(), free_uid)
 
     # Check uid-pool exhaustion
     #
-    # uid "-1" is locked now, so RequestUnusedUid is expected to fail
+    # free_uid is locked now, so RequestUnusedUid is expected to fail
     self.assertRaises(errors.LockError,
                       uidpool.RequestUnusedUid,
-                      set([-1]))
+                      set([free_uid]))
 
     # Check unlocking
     uid.Unlock()
     # After unlocking, "-1" should be available again
-    uid = uidpool.RequestUnusedUid(set([-1]))
-    self.assertEqualValues(uid.GetUid(), -1)
+    uid = uidpool.RequestUnusedUid(set([free_uid]))
+    self.assertEqualValues(uid.GetUid(), free_uid)
 
 
 if __name__ == "__main__":
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<div class='path'>path: <a href='/guix/log/?id=ee23adcc32cfd3b5ee6999adc965c2ca76feffbd'>root</a>/<a href='/guix/log/guix.scm?id=ee23adcc32cfd3b5ee6999adc965c2ca76feffbd'>guix.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/guix.scm?id=ee23adcc32cfd3b5ee6999adc965c2ca76feffbd&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-09-14 11:50:58 +0200'>2021-09-14</span></td><td><a href='/guix/commit/guix.scm?id=3858275962f1ac567b5dce2355035f9eddea0a05'>guix: Replace 'define-public'.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a followup to 8531997d2a1e10d574a6e9ab70bc86ade6af4733.

* guix.scm: Use #:re-export-and-replace for 'define-public'.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-06-29 00:00:13 +0200'>2021-06-29</span></td><td><a href='/guix/commit/guix.scm?id=e059adcb6001ab0353d969b24030d0c342f8830e'>(guix) no longer pulls in (guix ftp-client).</a><span class='msg-avail'>...<span class='msg-tooltip'>That choice was made in 2014 and never made much sense.

* guix.scm (%public-modules): Remove 'ftp-client'.
</span></span></td><td>Ludovic Courtès</td></tr>