assert-msg-test.py: Look for assert-msg-test relative to this script

When run as an installed-test, assert-msg-test generally won't be in
the PATH, but it will be in the same directory as the installed copy
of this script, so we can find it that way.

This fixes an installed-tests failure in Debian's autopkgtest
environment.

Signed-off-by: Simon McVittie <smcv@collabora.com>

glib/tests/assert-msg-test.py

@@ -64,7 +64,9 @@ class TestAssertMessage(unittest.TestCase):
                 os.environ["G_TEST_BUILDDIR"], "assert-msg-test"
             )
         else:
-            self.__assert_msg_test = shutil.which("assert-msg-test")
+            self.__assert_msg_test = os.path.join(
+                os.path.dirname(__file__), "assert-msg-test"
+            )
         print("assert-msg-test:", self.__assert_msg_test)
 
     def runAssertMessage(self, *args):