wip: act testing follow-up

[xnyo]

Part of #397

• Add a system for parsing of GHA commands, annotations, outputs, etc, in order to run assertions on them
• Add a system for mocking jobs and steps within jobs in the test workflows and workflows called from it
• Mock "Build and test" plugins in anything that doesn't test building or testing the plugin, to speed up act test runs
• Add pre-built simple-frontend testdata plugin to mocked data