gnu: grep: Skip timing-sensitive test.

This avoids failures on loaded machines as seen
at <http://hydra.gnu.org/build/906900/log/raw>.

* gnu/packages/patches/grep-timing-sensitive-test.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/base.scm (grep): Use it.

gnu-system.am

@@ -1,5 +1,5 @@
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2012, 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2012, 2013, 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
 # Copyright © 2013, 2014, 2015 Andreas Enge <andreas@enge.fr>
 # Copyright © 2013, 2014, 2015 Mark H Weaver <mhw@netris.org>
 #
@@ -498,6 +498,7 @@ dist_patch_DATA =						\
   gnu/packages/patches/gobject-introspection-absolute-shlib-path.patch \
   gnu/packages/patches/gobject-introspection-cc.patch		\
   gnu/packages/patches/gobject-introspection-girepository.patch	\
+  gnu/packages/patches/grep-timing-sensitive-test.patch		\
   gnu/packages/patches/grub-gets-undeclared.patch		\
   gnu/packages/patches/grub-freetype.patch			\
   gnu/packages/patches/guile-1.8-cpp-4.5.patch			\

gnu/packages/base.scm

@@ -80,7 +80,9 @@ command-line arguments, multiple languages, and so on.")
                                 version ".tar.xz"))
             (sha256
              (base32
-              "1srn321x7whlhs5ks36zlcrrmj4iahll8fxwsh1vbz3v04px54fa"))))
+              "1srn321x7whlhs5ks36zlcrrmj4iahll8fxwsh1vbz3v04px54fa"))
+            (patches
+             (list (search-patch "grep-timing-sensitive-test.patch")))))
    (build-system gnu-build-system)
    (native-inputs `(("perl" ,perl)))             ;some of the tests require it
    (synopsis "Print lines matching a pattern")

gnu/packages/patches/grep-timing-sensitive-test.patch

@@ -0,0 +1,15 @@
+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