Added the ability to filter which tests are run by name, using a --filter

lib/unittest/config.dart

Index: lib/unittest/config.dart
===================================================================
--- lib/unittest/config.dart	(revision 8995)
+++ lib/unittest/config.dart	(working copy)
@@ -61,7 +61,10 @@
     if (passed == 0 && failed == 0 && errors == 0) {
       print('No tests found.');
       // This is considered a failure too: if this happens you probably have a
-      // bug in your unit tests.
+      // bug in your unit tests, unless you are filtering.

[eub, 2012/06/21 20:58:45]

Is there a known use case where people want to filter down to zero tests and
have that silently succeed?  I can imagine it might exist but I don't see an
example.

[gram, 2012/06/21 21:18:03]

Here's the thing - we are constraining individual tests, not test files. So if
you have three test files in a directory A, B, C, and B contains test 'foo', and
you filter for 'foo', then files A and C will otherwise show up as 'test
failures'. Hence this change.

+      if (getFilter() != null) {
+        success = true;
+      }
     } else if (failed == 0 && errors == 0 && uncaughtError == null) {
       print('All $passed tests passed.');
       success = true;