Refactor AssertThrows to support Throwable

Prior to this commit, AssertThrows in the spring-test module only
supported Exception; however, there are legitimate test cases where the
subject under test (SUT) may potentially throw a subclass of Throwable.

This commit refactors AssertThrows so that it supports exceptions of
type Throwable instead of the limiting support for Exception.
Furthermore, AssertThrows has been refactored to use generics (e.g.,
Class<? extends Throwable> instead of merely Class).

Issue: SPR-6362