ulib/test-exceptions

This library provides utilities to handle exceptions in unittests. Expected exceptions can be registered which will catch and handle them appropriately, whereas unexpected exceptions or failing to catch a registered exception will trigger a test failure.

This replaced the RUN_TEST_ENABLE_CRASH_HANDLER mechanism that used to be provided by the unittest library, both of which have since been removed. The differences from that mechanism were:

• C++ object instead of macros
• synchronous behavior for simplicity and predictability
• not dependent on any particular test library so can be used equally well with the zxtest library or the now-removed unittest library
• uses new channel-based exception APIs