RFC/WIP: GDB Transparent support

This PR implements (WIP) transparent support for debugging applications inside a test.

The feature is intended to be useful for test runners, and not only test writers. That is, no changes to test code is necessary for debugging any application that runs inside the test.

If a condition is reached, such as unhandled signal or breakpoint, the user is going to be notified about that, and can step into the debugger during the "paused" test run.