test/api: check api.exception exits the process

The `api.exception` method, which is internally also used by
`api.exception_handler` helper, calls `sys.exit` and this no
statement after a call to either method should be reached.
Add an assertion to make sure of that.

test/mod/test_api.py

@@ -89,6 +89,7 @@ class TestAPI(unittest.TestCase):
         def exception(path):
             with osbuild.api.exception_handler(path):
                 raise ValueError("osbuild test exception")
+            assert False, "api.exception should exit process"
 
         api = osbuild.api.API(args, monitor, socket_address=path)
         with api: