Privacy Provider Tests by Default

[jrchamp]

Similar to #156, it would be good to trigger the core Privacy Provider tests in the standard GitHub Action/Travis workflows. Perhaps there's a whole collection of tests that are relevant to third party plugins that would be helpful in improving plugin quality.

[jrchamp]

Another test that hooks into plugin methods: core\externallib_test::test_all_external_info

Perhaps each of these tests could be "tagged" in core, imagine something like @plugin, and then those plugin-relevant tests could be run as part of the moodle-plugin-ci workflow.