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='2019-01-17 14:04:28 +0100'>2019-01-17</span></td><td><a href='/guix/commit/gnu/installer/newt/final.scm?id=938ee975af8f35ae16c15443a7a76be7d31278eb'>installer: Adjust messages on the final page.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/installer/newt/final.scm (run-config-display-page)
(run-install-success-page, run-install-failed-page): Adjust messages.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-01-17 14:04:25 +0100'>2019-01-17</span></td><td><a href='/guix/commit/gnu/installer/newt/final.scm?id=7d812901daf0259d5d381199168d6d2994ce00ac'>installer: Turn "Cancel" buttons into "Exit" buttons.</a><span class='msg-avail'>...<span class='msg-tooltip'>This change and previous ones were,
Suggested-by: Thorsten Wilms &lt;t_w_@freenet.de&gt;
here: https://lists.gnu.org/archive/html/guix-devel/2018-11/msg00330.html

gnu/installer/newt/ethernet.scm: Turn cancel into exit.
gnu/installer/newt/final.scm: Ditto.
gnu/installer/newt/keymap.scm: Ditto.
gnu/installer/newt/locale.scm: Ditto.
gnu/installer/newt/network.scm: Ditto.
gnu/installer/newt/page.scm: Ditto.
gnu/installer/newt/partition.scm: Ditto.
gnu/installer/newt/services.scm: Ditto.
gnu/installer/newt/timezone.scm: Ditto.
gnu/installer/newt/user.scm: Ditto.
gnu/installer/newt/wifi.scm: Ditto.
</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2019-01-17 14:04:22 +0100'>2019-01-17</span></td><td><a href='/guix/commit/gnu/installer/newt/final.scm?id=dc5f3275ecbddc804875899e9e457299a835d7ab'>installer: Add configuration formatter.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/installer.scm (installer-steps): Add configuration-formatter procedures.
* gnu/installer/final.scm: New file.
* gnu/installer/locale.scm (locale-&gt;configuration): New exported procedure.
* gnu/installer/newt.scm (newt-installer): Add final page.
* gnu/installer/newt/final.scm: New file.
* gnu/installer/record.scm (installer): Add final-page field.
* gnu/installer/timezone.scm (posix-tz-&gt;configuration): New exported
procedure.
* gnu/installer/steps.scm (installer-step): Rename configuration-proc field to
configuration-formatter.
(%installer-configuration-file): New exported parameter,
(%installer-target-dir): ditto,
(%configuration-file-width): ditto,
(format-configuration): new exported procedure,
(configuration-&gt;file): new exported procedure.
</span></span></td><td>Mathieu Othacehe</td></tr>