Avoid intermittent failures in KeystoreRealmTestCase

There have been intermittent test failures in KeystoreRealmTestCase on Windows with the following tests failing due to the test keystore being null:

KeystoreRealmTestCase.testUserRootConnection
KeystoreRealmTestCase.testUserPrincipalWithHeaders
KeystoreRealmTestCase.testRootConnectionNoCertificate
KeystoreRealmTestCase.testServerRootConnection
KeystoreRealmTestCase.testUserPrincipal

The setup for this test class attempts to assert that the test directory where the keystores will be created doesn't exist before attempting to create the keystores. If the test directory does happen to exist (because it's been left around by a different test class), the assertion will fail, preventing the keystores from being created.

We should fix the setup so that it deletes the test directory first if it happens to exist.