Fix issue in BasicSyntaxTest.short_name

mjtest/test/syntax_tests.py

@@ -29,7 +29,8 @@ class BasicSyntaxTest(TestCase):
             if self.file.startswith(subdir):
                 test_dir = subdir
                 break
-        assert test_dir != None
+        if test_dir is None:
+            return path.relpath(self.file, self.env.test_dir)
         return path.relpath(self.file, test_dir)
 
     def run(self) -> BasicTestResult: