Improve error reporting for nested asserts failures in the test suite.

Improve error reporting for nested asserts failures in the test suite.

If an assert occurred while handling an exception generated by a previous
assert handler, the information about the original assert was lost even if
though it was more important than the secondary assert. Do preserve and show
the original assert message in this case now.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71318 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775