test-application.c: Install a signal handler to exit cleanly

The app-test program gets run by atk_test_util.c, and gets sent a SIGTERM when the parent process wants it to terminate. This is fine, but if app-test terminates forcefully with the default handler for SIGTERM, then libgcc won't get a chance to write out its code coverage information.

So, we install a signal handler for SIGTERM, and the handler exits the main loop.

This increases test coverage from 33% to 57%, so yay!

Fixes #56 (closed)

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