Move common test functionality into package:test_common

[annagrin]

We have test utilities that we would like to use in several packages (dwds, webdev, etc). The share functionality lives currently in dwds/test/fixtures directory that is unaccessible to webdev to import from.

• Create a test_common package that we don't publish and move the shared utilities there:

  • logging
  • sdk_asset_generator
  • test sdk configuration

• After the package is created, re-enable weak null safety e2e tests in webdev.