Update mirgecom downstream testing to use integrated test runner.

[MTCam]

Use mirgecom integrated test runner to run lazy, examples, and production testing. Plugs a production testing hole.

NOTE: Currently production testing is disabled due to a snafu with the requirements.txt.

[inducer]

Thanks for this contribution.

Analogous CI code exists in, at least, arraycontext, pytato, and loopy. What's provided here looks a bit "non-final", and I'd prefer not to have to continue making changes in multiple places as things get finalized. Instead, I'd prefer if mirgecom had a script entrypoint for downstream tests, so that upstream package can defer the policy decision of what should constitute downstream CI can be deferred to mirgecom.