Narrow exception type when attempting to handle test failures. NFC (#20852)

`self.failureException` is the exception type that is thrown when a
test fails.

Author: sbc100

test/common.py

@@ -1638,7 +1638,7 @@ def _build_and_run(self, filename, expected_output, args=None, output_nicerizer=
             self.assertContained(expected_output, js_output, regex=regex)
             if assert_returncode == 0 and check_for_error:
               self.assertNotContained('ERROR', js_output)
-        except Exception:
+        except self.failureException:
           print('(test did not pass in JS engine: %s)' % engine)
           raise
     return js_output
@@ -1982,7 +1982,7 @@ def run_browser(self, html_file, expected=None, message=None, timeout=None, extr
           output = unquote(output)
           try:
             self.assertContained(expected, output)
-          except Exception as e:
+          except self.failureException as e:
             if extra_tries > 0:
               print('[test error (see below), automatically retrying]')
               print(e)