testsuite: Don't use sed -i in T15369
testsuite: Don't use sed -i in T15369

BSD sed doesn't allow use of sed -i on symlinks and the source file is symlinked
into place.

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