gnu: GHC: Adjust tests for grep 3.8.

This fixes a test failure introduced in commit
c527b0ce23.

* gnu/packages/patches/ghc-testsuite-grep-compat.patch: New file.
* gnu/local.mk (dist_patch_DATA): Adjust accordingly.
* gnu/packages/haskell.scm (ghc-8.6, ghc-8.8, ghc-8.10)[native-inputs]: Patch
the 'ghc-testsuite' input.

gnu/packages/patches/ghc-testsuite-grep-compat.patch

@@ -0,0 +1,39 @@
+Remove stray escapes to avoid warnings from grep 3.8 which breaks
+expected test output.
+
+diff --git a/testsuite/tests/hsc2hs/Makefile b/testsuite/tests/hsc2hs/Makefile
+--- a/testsuite/tests/hsc2hs/Makefile
++++ b/testsuite/tests/hsc2hs/Makefile
+@@ -47,5 +47,5 @@ T12504:
+ ifeq "$(WINDOWS)" "YES"
+ 	grep '{-# LINE 1 \"T12504\\\\path\\\\to\\\\$@\.hsc\" #-}' T12504/path/to/$@.hs
+ else
+-	grep '{-# LINE 1 \"T12504/path/to/$@\.hsc\" #-}' T12504/path/to/$@.hs
++	grep '{-# LINE 1 "T12504/path/to/$@\.hsc" #-}' T12504/path/to/$@.hs
+ endif
+diff --git a/testsuite/tests/numeric/should_run/T7014.primops b/testsuite/tests/numeric/should_run/T7014.primops
+--- a/testsuite/tests/numeric/should_run/T7014.primops
++++ b/testsuite/tests/numeric/should_run/T7014.primops
+@@ -1,8 +1,8 @@
+ and#
+ or#
+ uncheckedShift.*#
+-\+#
+-\-#
+++#
++-#
+ \*#
+ quotInt#
+ remInt#
+diff --git a/testsuite/tests/simplCore/should_compile/Makefile b/testsuite/tests/simplCore/should_compile/Makefile
+--- a/testsuite/tests/simplCore/should_compile/Makefile
++++ b/testsuite/tests/simplCore/should_compile/Makefile
+@@ -226,7 +226,7 @@ str-rules:
+ # g should have been collapsed into one defininition by CSE.
+ .PHONY: T13340
+ T13340:
+-	'$(TEST_HC)' $(TEST_HC_OPTS) -c -O T13340.hs -ddump-simpl -dsuppress-all | grep -c '\+#'
++	'$(TEST_HC)' $(TEST_HC_OPTS) -c -O T13340.hs -ddump-simpl -dsuppress-all | grep -c '+#'
+ 
+ 
+ # We expect to see all dictionaries specialized away.