Make test.py pick migrated suites by default when the `NnbdMode` is not `NnbdMode.legacy`.
* Remove non-existing benchmark_smoke suite.
* Remove broken analyzer_library suite from default suites.
* Deprecated observatory_ui is not added to the default migrated suites.
* Remove remaining references to samples-dev.
* Remove unnecessary suite specifications from the test_matrix.json.
* Remove broken references to observatory_ui from the test_matrix.json.
* Remove defunct observatory_ui hack from test.py.
Fixes: b/268474066
Change-Id: I6c3635bcb396021ca86498d60c9efc77ab39589e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/281701
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
Alexander Thomas
b9d8103371