gnu: ghc-9.2: Adapt testsuite to grep 3.8.

Starting with version 3.8, grep emits warning messages when pattern
contain spurious slashes:

    grep '\+' /dev/null
    grep: warning: stray \ before +

GHC’s testsuite does not expect that and fails.

* gnu/packages/patches/ghc-9.2-grep-warnings.patch: New file.
* gnu/local.mk: Add it.
* gnu/packages/haskell.scm (ghc-9.2)[native-inputs]: Add patch.

gnu/packages/haskell.scm

@@ -1354,7 +1354,8 @@ interactive environment for the functional language Haskell.")
                     version "/ghc-" version "-testsuite.tar.xz"))
              (sha256
               (base32
-               "19ha0hidrijawy53vm2r0sgml5zkl8126mqy7p0pyacmw3k7913l"))))
+               "19ha0hidrijawy53vm2r0sgml5zkl8126mqy7p0pyacmw3k7913l"))
+             (patches (search-patches "ghc-9.2-grep-warnings.patch"))))
          ,@(filter (match-lambda
                      (("ghc-bootstrap" . _) #f)
                      (("ghc-testsuite" . _) #f)