parser: add support for returning just basename in clang diagnostics

This will be useful for parser tests, as we'll be able to reuse this
instead of duplicating the logic for filename or line being None.

src/hawkmoth/parser.py

@@ -65,12 +65,14 @@ class ParserError:
     line: int
     message: str
 
-    def get_message(self):
+    def get_message(self, basename=False):
         if self.filename:
+            filename = os.path.basename(self.filename) if basename else self.filename
+
             if self.line is not None:
-                return f'{self.filename}:{self.line}: {self.message}'
+                return f'{filename}:{self.line}: {self.message}'
             else:
-                return f'{self.filename}: {self.message}'
+                return f'{filename}: {self.message}'
         else:
             return f'{self.message}'