UnitTest.exe runs on CI and from tooling, where an assert or an abort() puts
up a message box that nothing will ever click, and the run just hangs until it
is killed. Headless already solved this; move its SetupCRT() into Common
(ExceptionHandlerSetup, which is where the rest of the process-level fault
setup lives) and call it from the unit tests too.
No behaviour change for headless. The OS-level SetErrorMode() call is now
guarded for UWP, which doesn't have it.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GZq8ZtJmFY7bkX5FVkr3P9
