Add a skip to an empty test group (flutter#5080)

The empty group will soon become a warning or an error. Eagerly skip the
group to avoid any issue when the test runner changes behavior.
dart-lang/test#2012

Note that the linked issue has been resolved, and the comment in this
group body may be out of date.

This change will impact the output from running tests - there will be an
indication of the skipped test.

packages/flutter_markdown/test/emphasis_test.dart

@@ -4404,7 +4404,7 @@ void defineTests() {
         //
         // Test for rule 17 are not included since markdown package is not
         // following the rule.
-      });
+      }, skip: 'No Rule 17 tests implemented');
     },
   );
 }