Share common WTR config across packages

[emensch]

Currently, each package defines its own config file for web-test-runner and many of them are duplicates. Additionally, each testing dependency must be installed per-project. Ideally, we should share the "base" config and dependencies across packages and allow each to extend according to their specific requirements.

This may be a use-case for Rush rig packages.

[crandmck]

@adobe export issue to Jira project CAI

[github-jira-sync-bot]

:white_check_mark: Jira issue https://jira.corp.adobe.com/browse/CAI-3378 is successfully created for this GitHub issue.