extending tests to non-code-generation

[ksjogo]

Currently only the actual code generation is tested.
We should look into creating/having more tests to have higher confidence in changing project-level scope.
#57 is preparing that somewhat with using the real project class to be used in the tests, but some explicit tests not related to codegen could be good.
We might need another test runner for that. And potentially move the generation tests also to that runner.
I will look into that after #57, but am interested to hear thoughts / ideas / opinions.

[johnfn]

I would LOVE this. It's been on my TODO list for so long.

Some way to mock out file/directory structure, and ensure that ts2gd does the right thing (generates node paths correctly, generates files in the right place etc) would be AMAZING.

We might have to use jest or something for this, yeah.