Fix file contention in TestTileFromCommits
Fix file contention in TestTileFromCommits

BUG=skia:5278

Review-Url: https://codereview.chromium.org/1961963004

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot