cases: changed tests to be more version-aware

Removed Skips introduced wit 13252.

Verified with all three possible permutations of compiler/package
version difference.

For golang/go#11811.
Fixes golang/go#11927.

Change-Id: I9c607eb5b27721384559c1d96fc49e73079b365b
Reviewed-on: https://go-review.googlesource.com/13300
Reviewed-by: Russ Cox <rsc@golang.org>
1 file changed