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
td><td class='form'><form method='get'>
<input type='hidden' name='id' value='17375266ea2e94726bf3d28c1d4fb3899c115377'/><select name='h' onchange='this.form.submit();'>
<option value='koszko' selected='selected'>koszko</option>
<option value='koszko-scripts'>koszko-scripts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=17375266ea2e94726bf3d28c1d4fb3899c115377'>refs</a><a class='active' href='/guix/log/tests/upstream.scm'>log</a><a href='/guix/tree/tests/upstream.scm?id=17375266ea2e94726bf3d28c1d4fb3899c115377'>tree</a><a href='/guix/commit/tests/upstream.scm?id=17375266ea2e94726bf3d28c1d4fb3899c115377'>commit</a><a href='/guix/diff/tests/upstream.scm?id=17375266ea2e94726bf3d28c1d4fb3899c115377'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/upstream.scm'>
<input type='hidden' name='id' value='17375266ea2e94726bf3d28c1d4fb3899c115377'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=17375266ea2e94726bf3d28c1d4fb3899c115377'>root</a>/<a href='/guix/log/tests?id=17375266ea2e94726bf3d28c1d4fb3899c115377'>tests</a>/<a href='/guix/log/tests/upstream.scm?id=17375266ea2e94726bf3d28c1d4fb3899c115377'>upstream.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/upstream.scm?id=17375266ea2e94726bf3d28c1d4fb3899c115377&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-01-05 21:17:58 +0100'>2022-01-05</span></td><td><a href='/guix/commit/tests/upstream.scm?id=6226df77ee7448ba4eae05c14aa2ba9721f28bb4'>tests: Add failing tests for changed-inputs on new style inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>* tests/upstream.scm ("changed-inputs returns changes to plain input list",
"changed-inputs returns changes to all plain input lists"): New tests.
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2022-01-05 21:17:54 +0100'>2022-01-05</span></td><td><a href='/guix/commit/tests/upstream.scm?id=a6bf1c108ef3b0d63d6d6070b90093bc8f434734'>tests: Add tests for changed-inputs on old-style inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>All these tests pass, because they only test the old-style input alists with
labels.

* tests/upstream.scm ("changed-inputs returns no changes",
"changed-inputs returns changes to labelled input list",
"changed-inputs returns changes to all labelled input lists"): New tests.
</span></span></td><td>Ricardo Wurmus</td></tr>