[stable][dart2js] Update tests matrix entries

- Update options for existing configurations.
- Add one new configuration for running _2 test suites.
- Rename builders and update to run new configurations.

Tested: Tested with led runs.
Issue: #51481
Cherry-pick: https://dart-review.googlesource.com/c/sdk/+/281348
Change-Id: I695525b0bfa936ad6c5c7f2ecac212299de969c2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/287903
Reviewed-by: Alexander Thomas <athom@google.com>

runtime/observatory/tests/observatory_ui/observatory_ui.status

@@ -5,6 +5,9 @@
 [ !$browser ]
 *: SkipByDesign
 
+[ $compiler == dart2js && $runtime == chrome ]
+*: Slow, Pass # https://github.com/dart-lang/sdk/issues/51651
+
 # custom elements are not supported on old browsers, we don't
 # intend for observatory to work on old browser versions, so
 # skipping.