Better ABORT_SLIM_TEST handling and 'StopSuite' support

Modified the handling of <throw 'StopTest'> so that it matches the
FitNesse expection: the test page halts but the suite is not aborted.
Additionally, added support for <throw 'StopSuite'> so that a user can
halt suite execution.

Other changes:
* Made exception tests v2 compatible.
* Added <throw 'StopSuite'> test case.