pam-srv-tests: Reuse test directory for IO tests
    
    This patch is related to commit 50c9d542e8bf641412debaa82a4dcf67ddb72258
    "tests: Use unique name for TEST_PATH"
    
    It's better to do IO operation in common test directory
    to prevent conflict with other test (copy & paste errors)
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>