Isolate testing Penlight with busted from Busted using Penlight

[alerque]

So my brilliant idea in #356 to use Busted for the test suite has an unexpected consequence! I just spent an hour chasing my tail trying to run tests and getting bizarre results before it finally dawned on my why it was breaking in places I hadn't touched. Busted itself requires Penlight for internal use. 🤦

The result of course is a dependency race condition. It's not always clear what version of Penlight is being used where! It works okay testing Penlight modules that Busted has not already loaded, but even manually setting the package.path is useless for testing modules that Busted already loaded for its own use, all you get is the cached version! Conversely it's possible to convince Busted to load the local code, but then a broken module can actually break the Busted runner itself rather than failing the respective test.

I'm not quite sure how to untangle this so that both local and CI test runs reliably ⓐ use a stable version of Penlight to run Busted and ⓑ test the Penlight module code as found in the local working directory.

[alerque]

It seems like Busted's insulate() and particular the --auto-insulate which defaults to on should be taking care of this. I'm kind of confused why it is not, this could be a Busted bug.

[Tieske]

discussion moved to lunarmodules/busted#643