guard xml-using tests with SK_XML

I think this will fix the Google3 roll.

Unclear to me if we actually want this to work there.
I guess it comes down to the expat dependency?

Change-Id: I5f8a6263aeae29e338a13207845fbe51ac9acc18
Reviewed-on: https://skia-review.googlesource.com/104562
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed