test: handle clang diagnostics returning None for filename

The parser may return None for clang diagnostics filename if there isn't
one. We already handle this gracefully in get_message(), so reuse that
instead of duplicating the logic, now that there's support for
basename=True.

test/test_parser.py

@@ -2,8 +2,6 @@
 # Copyright (c) 2018-2023, Jani Nikula <[email protected]>
 # Licensed under the terms of BSD 2-Clause, see LICENSE for details.
 
-import os
-
 import pytest
 
 from hawkmoth import docstring
@@ -81,7 +79,7 @@ def get_output(self):
             roots[key] = root
 
             for error in errors:
-                errors_str += f'{error.level.name}: {os.path.basename(error.filename)}:{error.line}: {error.message}\n'  # noqa: E501
+                errors_str += f'{error.level.name}: {error.get_message(basename=True)}\n'
 
         for directive in self.directives:
             filename = directive.get_input_filename()