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='2018-10-01 12:12:20 +0200'>2018-10-01</span></td><td><a href='/guix/commit/gnu/packages/idris.scm?id=53812aa77f5c8a490191c069a13a927d365618c7'>gnu: idris: Update to 1.3.0.</a><span class='msg-avail'>...</span></td><td>Timothy Sample</td></tr>
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<tr><td><span title='2018-03-15 23:46:16 +0100'>2018-03-15</span></td><td><a href='/guix/commit/gnu/packages/idris.scm?id=d661ed521eedf4663c57e5d96e409a32a52113e0'>gnu: idris: Provide the right versions of dependencies.</a><span class='msg-avail'>...</span></td><td>Alex ter Weele</td></tr>
<tr><td><span title='2018-03-15 23:46:16 +0100'>2018-03-15</span></td><td><a href='/guix/commit/gnu/packages/idris.scm?id=866b19086e7f294261e694b228ff05ffb129296c'>gnu: Add ghc-cheapskate-0.1.0.5.</a><span class='msg-avail'>...</span></td><td>Alex ter Weele</td></tr>
<tr><td><span title='2018-03-15 23:46:15 +0100'>2018-03-15</span></td><td><a href='/guix/commit/gnu/packages/idris.scm?id=78dc985bee57eaaa45cb9b38c93c8ac4148e908d'>gnu: Add ghc-trifecta-1.6.2.1.</a><span class='msg-avail'>...</span></td><td>Alex ter Weele</td></tr>
<tr><td><span title='2018-03-15 23:46:15 +0100'>2018-03-15</span></td><td><a href='/guix/commit/gnu/packages/idris.scm?id=9c7e17fd61826506a06b2bf737ebd8bbcd0ce2b5'>gnu: Add ghc-aeson-1.1.2.0.</a><span class='msg-avail'>...</span></td><td>Alex ter Weele</td></tr>
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<tr><td><span title='2017-11-13 18:12:27 +0100'>2017-11-13</span></td><td><a href='/guix/commit/gnu/packages/idris.scm?id=a06b9b50075a78f846093f86da85d5806b36fbf7'>gnu: Move testing packages from haskell to haskell-check.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>