Make the testutils API public
We have a nice internal API for testing: https://gitlab.com/BuildStream/buildstream/tree/master/tests/testutils
This API has very neat helpers for testing source and element plugins, but since it's not in the buildstream module it can only be accessed from within our codebase - this is a bit of a waste, plugin developers would probably be interested in using these helpers.
We should clean and open up this API (perhaps move it into the buildstream module) - this way we could also use it in our bst-external repo without copying the module, which would keep the test suites in sync.