The test for reading old records should only run against filesystem stores

The test (com.hp.mwtests.ts.arjuna.objectstore.TestJBTM3262) was added while fixing https://issues.redhat.com/browse/JBTM-3262 in order to verify that records written with the old string serialization code could be read by later revisions of that code. Since the same string serialization code is used for all store types we only need to test against a single store type (adding the same test for all store types would be overkill).

The test is coded to work with stores that inherit from ObjectStore (such as ShadowNoFileLockStore) but it fails with some store types such as the JDBCStore with the error:

TestJBTM3249.test:86 com.arjuna.ats.internal.arjuna.objectstore.jdbc.JDBCStore cannot be cast to com.arjuna.ats.arjuna.objectstore.ObjectStore