Use random names for test files
Use random names for test files

GCS has a rate-limit on changes on objects.
The test suite used to fail because we were running all the tests on the same 'foo' & 'bar' objects and hitting those rate-limits.

Test filenames are now random, so it no longer happens.

Fixes #34

renchap/shrine-google_cloud_storage,renchap/shrine-google_cloud_storage