tests/tcg: Stop using exit() in the gdbstub testcases

tests/tcg: Stop using exit() in the gdbstub testcases

GDB 15 does not like exit() anymore:

    (gdb) python exit(0)
    Python Exception <class 'SystemExit'>: 0
    Error occurred in Python: 0

Use the GDB's own exit command, like it's already done in a couple
places, everywhere. This is the same fix as commit 93a3048dcf45
("tests: Gently exit from GDB when tests complete"), but applied to
more places.

Acked-by: Gustavo Romero <gustavo.romero@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20241022113939.19989-1-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>