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='2018-01-28 23:14:55 +0100'>2018-01-28</span></td><td><a href='/guix/commit/scripts/guix.in?id=63666a348af5ef5190cbe9611c40f2b9188b7967'>guix: Let Emacs detect “scripts/guix.in” appropriate mode.</a><span class='msg-avail'>...</span></td><td>Mathieu Lirzin</td></tr>
<tr><td><span title='2018-01-23 15:01:15 +0100'>2018-01-23</span></td><td><a href='/guix/commit/scripts/guix.in?id=6f774d481839f87178c5895ac2d661e141f879b8'>guix: Refactor script.</a><span class='msg-avail'>...</span></td><td>Mathieu Lirzin</td></tr>
<tr><td><span title='2018-01-23 10:13:16 +0100'>2018-01-23</span></td><td><a href='/guix/commit/scripts/guix.in?id=cba386c1295b7054df22dfb4662b1b92f668e2c0'>build: Expand ‘scripts/guix’ at Make time.</a><span class='msg-avail'>...</span></td><td>Mathieu Lirzin</td></tr>
<tr><td><span title='2017-03-20 23:26:18 +0100'>2017-03-20</span></td><td><a href='/guix/commit/scripts/guix.in?id=9437fd7399425ed281e1bfc13240288ba9078caa'>build: Install .go files to $libdir/guile/X.Y.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-04-06 23:21:26 +0200'>2016-04-06</span></td><td><a href='/guix/commit/scripts/guix.in?id=7cffaeb60473e25a7ef8c1a8cb36e1531191873a'>challenge: Really exit with non-zero upon hash mismatch.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2014-06-14 22:55:59 +0200'>2014-06-14</span></td><td><a href='/guix/commit/scripts/guix.in?id=243cea2461e3f260cef2c702689252a54858fdfc'>guix: Look for "guix-packages" messages in $localedir.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>