Central place with a list of queries for transformer tests

This is most definitely for another PR, but as a general question (to whomever happens to read this), would it not be beneficial for future parser developers/contributors that we have a central place with a list of queries, wherein one could also add their parser's expected output for testing? E.g. a python dictionary or importable JSON file?

_Originally posted by @CasperWA in https://github.com/Materials-Consortia/optimade-python-tools/pull/66#discussion_r345824371

Response by @fekad:

That's a good idea. But rather having a dictionary I would prefer a python template file which contains all the cases. The dictionary/JSON can easily become quite complex when you start to categorise the test cases or you want to test failures with different exceptions etc...

Originally posted by @fekad in https://github.com/Materials-Consortia/optimade-python-tools/pull/66#discussion_r346109975