Only run image tests on Windows once

The Windows filesystem is case insensitive, so by asking for all .xxx
files and then all .XXX files, we ended up processing every file twice.

Bug: skia:
Change-Id: I2b9e8bff988eefa32c40e58eef5b297cd0166231
Reviewed-on: https://skia-review.googlesource.com/152382
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed