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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<tr><td><span title='2022-05-23 16:24:46 +0200'>2022-05-23</span></td><td><a href='/guix/commit/tests/guix-package.sh?id=aaf547824e389b68c028c9b3fd4d15fd2991dc31'>scripts: package: Transform before creating manifest entries.</a><span class='msg-avail'>...</span></td><td>Josselin Poiret</td></tr>
<tr><td><span title='2022-02-18 14:14:38 +0100'>2022-02-18</span></td><td><a href='/guix/commit/tests/guix-package.sh?id=5a57313918a5c1bd5cf8238c2dcae64a03523952'>profiles: 'profile-derivation' rejects unsupported packages.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-04-29 01:22:04 +0200'>2021-04-29</span></td><td><a href='/guix/commit/tests/guix-package.sh?id=524c9800afb433cc474132185d8e37f72004adb3'>diagnostics, ui: Adjust to 'read-error' and 'syntax-error' in Guile 3.0.6.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-03-05 12:49:27 +0100'>2021-03-05</span></td><td><a href='/guix/commit/tests/guix-package.sh?id=90ea8b16eb519a88d8f739fea5a416c0b99de19f'>profiles: 'package-&gt;manifest-entry' preserves transformations by default.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-02-01 17:32:35 +0100'>2021-02-01</span></td><td><a href='/guix/commit/tests/guix-package.sh?id=9fd7b050e2198c89f0763fe79f5c541b77b8e5c9'>guix package: Add '--export-manifest'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>