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
delete mysterious failures

junit.framework.AssertionFailedError: Exception thrown: java.lang.NullPointerException:
null
	at junit.framework.Assert.fail(Assert.java:57)
	at junit.framework.TestCase.fail(TestCase.java:227)
	at org.bouncycastle.pqc.jcajce.provider.test.FlexiTest.fail(FlexiTest.java:64)
	at org.bouncycastle.pqc.jcajce.provider.test.KeyPairGeneratorTest.performKeyPairEncodingTest(KeyPairGeneratorTest.java:43)
	at org.bouncycastle.pqc.jcajce.provider.test.McElieceCCA2KeyPairGeneratorTest.testKeyPairEncoding_9_33(McElieceCCA2KeyPairGeneratorTest.java:34)