Allow tests to run in any TZ

StubEntitlementCertificates were setting the default
start/end dates in localtime which would cause tests
to fail if local TZ was GMT + >= 1 hour.

Certificate.is_valid() compares against UTC now which
fails against a TZ as specified above.
Merge pull request #969 from candlepin/mstead/tests-run-in-any-tz

Allow tests to run in any TZ