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.