ITS: Increase test coverage

* Add a test case to check that setting data that exceeds maximum
  allowed size returns a failure.
* Modify existing test case to get after two consecutive sets
  to check that the the most recent flag set is returned.

Signed-off-by: Awadhy Mohammed <awadhy.mohammed@arm.com>
Change-Id: I1c0e30d4aaac640d042f289bc2b12a2e64504758
4 files changed