Make tests for folders with 500+ files

[mohamedsalem401]

We need to add tests for large folders: https://github.com/datopian/markdowndb/issues/22

I think we should make a new repository for that

[dstengle]

You could just generate your import data for this one test until you find a test case that is more complex.

Just make generator that creates some directories of different levels and different files, assert that that there are actually > 500 created and then run the test.