More intelligent CI tests

[ewels]

Currently, if there's a problem with a module then I request that some example data be added to the MultiQC_TestData repository for the CI tests. This is great when fixing that problem, but has the side effect of making all other commits and PRs fail CI until that fix is merged.

It would be nice to look at the diff in a given CI run to see which files are being modified. If only a single module is edited, we could run just that module in the tests. Then other PRs would not fail, and the tests would be faster. If any core MultiQC files are edited we can still run everything.