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='2019-01-20 19:21:43 +0100'>2019-01-20</span></td><td><a href='/guix/commit/gnu/packages/animation.scm?id=e5ad2cdf172eecc7edef37a500593b1941af013c'>gnu: Add pencil2d.</a><span class='msg-avail'>...</span></td><td>Pkill -9</td></tr>
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