Fix GlobusGSSContextTest

The GlobusGSSContextTest relies on having a credential, which then relies
on X509Credential class not having a cached copy of such a credential.
However, it does not ensure there is no such cached credential.

Since multiple tests (and test-suites) are run within the same JVM
instance, the effect is that, if another test-suite caches a
(deliberately) invalid, self-signed certificate then this cached
copy is returned instead of the expected certificate.

This patch fixes this problem by ensuring the cache is cleared.  It
introduces a (negliable) overhead as the cache is flushed between
tests; however, the additional robustness justifies this cost.

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>