devtools/lib/testutil: only copy xunit file to gcloud when it exists.
devtools/lib/testutil: only copy xunit file to gcloud when it exists.

Change-Id: Idc1e3127d5bd8efbf0030e58a82ea5fd79f83f00

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools