testsuite: Mark OldModLocation as broken on Windows

Strangely the path it emits contains duplicate path delimiters (#16772),
```patch
--- ghc-api/downsweep/OldModLocation.run/OldModLocation.stderr.normalised	2019-06-04 14:40:26.326075000 +0000
+++ ghc-api/downsweep/OldModLocation.run/OldModLocation.run.stderr.normalised	2019-06-04 14:40:26.328029200 +0000
@@ -1 +1 @@
-[Just "A.hs",Just "mydir/B.hs"]
+[Just "A.hs",Just "mydir//B.hs"]
```