TestSuiteTest: Generate more unique result file names. Speculative fix for flaky runs of TestSuiteTest. Flakes appear to read the results of other tests suggesting rand is not unique enough to protect against tests generating the same results file. Bug: chromium:1352134 Change-Id: Icadf64c557e23e6d87fd18aa60aa48373ecec9fd Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3827144 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>