core: fix tests by catching unhandled UndeclaredVariableWarnings

[LinqLover]

This is necessary to complement SUnit-ct.130, which treats unhandled warnings as a failure of the test case. Note that there are still some other things to do at the moment:

• This particular exception is weird at all - a warning should not "do nothing" in its defaultAction, so maybe the superclass of this exception should be changed instead
• smalltalkCI has not yet been told about the new semantics of warnings in tests (I'm on it).

Given these points, I'm parking this patch here for now until the above points have been resolved ...

Depends on #92.

[tom95]

Merging temporarily to get the CI to be useful again :)

Will be reverted if needed.