Don't throw assert failure exception in the test suite if it's unsafe.

Don't throw assert failure exception in the test suite if it's unsafe.

Don't throw when already handling an exception as it would result in a call to
terminate() and no useful information about the test failure would be given.
Abort ourselves instead to at least give the message about the assert failure.

This should help debug the mysterious ListCtrlTestCase failures in buildbot
wxGTK builds.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65760 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775