Add support for running additional tests for adjacent projects (canonical and feature plugins) #253
Description
Is your feature request related to a problem? Please describe.
Given that there are Canonical Plugins that are officially recommended and supported by the Core team/surrounding communities as well as feature plugins that aspire to be merged into Core one day, it would be great to add support for running the unit tests for these plugins in addition to the test suite for Core itself.
Describe the solution you'd like
The runner could accept a list of plugins to run tests for after running the Core test suite.
WPT_TEST_PLUGINS="wordpress-importer,gutenberg"
Additional context
This may not be possible if the setup required for each plugin is inconsistent without some level of coordination.
The reporter would also need to be updated to consume the additional data.
Props to @getsource for this idea during the review of #249.