tests: Gently exit from GDB when tests complete

tests: Gently exit from GDB when tests complete

GDB commit a207f6b3a38 ('Rewrite "python" command exception handling')
changed how exit() called from Python scripts loaded by GDB behave,
turning it into an exception instead of a generic error code that is
returned. This change caused several QEMU tests to crash with the
following exception:

Python Exception <class 'SystemExit'>: 0
Error occurred in Python: 0

This happens because in tests/guest-debug/test_gdbstub.py exit is
called after the tests have completed.

This commit fixes it by politely asking GDB to exit via gdb.execute,
passing the proper fail_count to be reported to 'make', instead of
abruptly calling exit() from the Python script.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240515173132.2462201-4-gustavo.romero@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>