mtest: limit "magic" CTRL+C behavior to process group leaders

If meson is not a process group leader, a SIGINT will be delivered also to
its parent process (and possibly other processes).  The parent process then
will probably exit and mtest will continue running in the background, without
any way to interrupt the run completely.

To fix this, treat SIGINT and SIGTERM the same way unless mtest is a
process group leader.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

mesonbuild/mtest.py

@@ -1893,7 +1893,10 @@ class TestHarness:
             l.start(self)
 
         if sys.platform != 'win32':
+            if os.getpgid(0) == os.getpid():
                 asyncio.get_event_loop().add_signal_handler(signal.SIGINT, sigint_handler)
+            else:
+                asyncio.get_event_loop().add_signal_handler(signal.SIGINT, sigterm_handler)
             asyncio.get_event_loop().add_signal_handler(signal.SIGTERM, sigterm_handler)
         try:
             for runner in runners: