Backport fragile testsuite modifier to ghc-8.8

I originally wasn't intending on backport this but my hand was forced:

• in !710 (merged) (for master) we mark a test in unix as fragile
• we want to continue using unix-2.7 on GHC's master branch (since otherwise we would need to bump the unix bound in Cabal and several others)
• consequently we must backport fragile if we want to continue using unix-2.7 in both ghc-8.8 and master