Fix for AS7-6616 - Make sure the unit test sets up the NamingStore

The commit here fixes the issue reported in https://issues.jboss.org/browse/AS7-6616. The testcase was failing because a different testcase (NamingSubsystemTestCase) ends up running before the InitialContextTestCase and the NamingSubsystemTestCase ends up setting a static variable which InitialContextTestCase relies on, to null.

The fix here makes sure that the InitialContextTestCase sets up the naming store appropriately before running the tests.