Refactor groups to pipe them through to the runner.

lib/src/runner.dart

Index: lib/src/runner.dart
diff --git a/lib/src/runner.dart b/lib/src/runner.dart
index 9a5756bc6de3f9cd4727205c90e47c6cb89e1422..e5831e58a8d29d6d44921f6ec68b54ea8f9253df 100644
--- a/lib/src/runner.dart
+++ b/lib/src/runner.dart
@@ -159,8 +159,7 @@ class Runner {
     })).map((loadSuite) {
       return loadSuite.changeSuite((suite) {
         if (_config.pattern == null) return suite;
-        return suite.change(tests: suite.tests.where((test) =>
-            test.name.contains(_config.pattern)));
+        return suite.filter((test) => test.name.contains(_config.pattern));
       });
     });
   }