testsuite: Mark T15633a and T15633b as fragile on Windows
testsuite: Mark T15633a and T15633b as fragile on Windows

As noted in #16813, these tests seem to be fragile on Windows.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc