Skip this performance regression test.

The test measures things on the order of 20ms.  On a loaded machine, we
have seen enough variation that the test would fail.

--- grep-2.22/tests/long-pattern-perf	2016-01-03 12:52:38.491575007 +0100
+++ grep-2.22/tests/long-pattern-perf	2016-01-03 12:53:39.768464687 +0100
@@ -16,6 +16,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+exit 77
 . "${srcdir=.}/init.sh"; path_prepend_ ../src
 
 fail=0
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<div class='path'>path: <a href='/guix/log/?id=37c2e94cec6cb8b5e0e93e7b6c712c3b187ca5db'>root</a>/<a href='/guix/log/tests?id=37c2e94cec6cb8b5e0e93e7b6c712c3b187ca5db'>tests</a>/<a href='/guix/log/tests/discovery.scm?id=37c2e94cec6cb8b5e0e93e7b6c712c3b187ca5db'>discovery.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/tests/discovery.scm?id=37c2e94cec6cb8b5e0e93e7b6c712c3b187ca5db&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2017-07-03 23:51:23 +0200'>2017-07-03</span></td><td><a href='/guix/commit/tests/discovery.scm?id=960c6ce96d746cf19829ad26e092ec5dad2a5c62'>discovery: Recurse into directories pointed to by a symlink.</a><span class='msg-avail'>...<span class='msg-tooltip'>Reported by Christopher Baines &lt;mail@cbaines.net&gt;
and Alex Kost &lt;alezost@gmail.com&gt;
at &lt;https://lists.gnu.org/archive/html/guix-devel/2017-06/msg00290.html&gt;.

* guix/discovery.scm (scheme-files): When ENTRY is a symlink that
doesn't end in '.scm', call 'stat' and recurse if it points to a
directory.
* tests/discovery.scm ("scheme-modules recurses in symlinks to
directories"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-06-18 00:14:07 +0200'>2017-06-18</span></td><td><a href='/guix/commit/tests/discovery.scm?id=d46c4423f46278bd2f96770ceb0667431414349e'>discovery: 'scheme-files' returns '() for a non-accessible directory.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes a regression introduced in
d27cc3bfaafe6b5b0831e88afb1c46311d382a0b.

Reported by Ricardo Wurmus &lt;rekado@elephly.net&gt;.

* guix/discovery.scm (scheme-files): Catch 'scandir*' system errors.
Return '() and optionally raise a warning upon 'system-error'.
* tests/discovery.scm ("scheme-modules, non-existent directory"): New
test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-05-03 23:50:15 +0200'>2017-05-03</span></td><td><a href='/guix/commit/tests/discovery.scm?id=cd903ef7871170d3c4eced45418459d293ef48a7'>Add (guix discovery).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/discovery.scm, tests/discovery.scm: New files.
* gnu/packages.scm (scheme-files, file-name-&gt;module-name)
(scheme-modules, all-package-modules): Remove.
(fold-packages): Rewrite in terms of 'fold-module-public-variables'.
* gnu/tests.scm: Use (guix discovery).
* Makefile.am (MODULES): Add guix/discovery.scm.
(SCM_TESTS): Add tests/discovery.scm.
</span></span></td><td>Ludovic Courtès</td></tr>