[FEATURE] Run integreation tests in parallel

[shonfeder]

Currently we run all the integration tests as one long sequence. But they are all completely independent, so we should be able to run them in parallel.

This can help reduce the friction in our build/test cycle in speed up our CI, addressing #172 .

[shonfeder]

I think the easy path to this would proceed as follows:

• [ ] Factor the current integration tests into separate files
• [ ] Add a makefile pattern target for all test/*.md files to run each with them with mdx-test.py.
• [ ] Change promote target to update all those MD files
• [ ] Change integration target to run all the integration file tests
• [ ] Add each integration test file name to the build matrix of the CI job, and run a separate build for each file.