Set up unit testing

[ewels]

When we go through refactoring modules for v2.0, take the opportunity to also set up a proper unit testing framework.

Can dump more ideas here, but for example:

• For each module, run a core set of tests:
  • On init, are mandatory variables set
  • On parse + ignore, does it return an exception because no data
  • ...

We can keep the main integration test of running MultiQC on everything, but a bunch of the additional tests that we currently have in the CI can almost certainly be better done as unit tests.

We also have various other sporadic mini-test scripts etc all over the place, so can consolidate those.

[vladsavelyev]

Regarding the currently existing integration testing, there are several modules that fail when multiple incompatible inputs are passed (preseq, bclconvert), and they probably should make MultiQC exit with error code 1. It's not practical at the moment because that would make the integration test multiqc test_data/modules always fail, since we have mixture of inputs for those modules in MultiQC_TestData.

So I think it might makes sense to break up those modules into module groups, using the config with path_filters in the CI runs.