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='2019-05-31 20:22:23 +0100'>2019-05-31</span></td><td><a href='/guix/commit/gnu/tests/web.scm?id=2177d9222f8c228fe5cd4e9c98d96f97e9601b86'>services: Add patchwork.</a><span class='msg-avail'>...</span></td><td>Christopher Baines</td></tr>
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