Format cert-revocation date in GMT to avoid timezone-dependent tests

In my London-based JVM, the date was actually Apr 19th, and the
asserted string in NistCertPathTest did not match.

This is an alternative to this prior pull-request, which just relaxed
the test condition to ignore the date:

https://github.com/bcgit/bc-java/pull/45#issuecomment-31311148