Improve the way we're organizing operator tests

[elboletaire]

We now can test our operators, but using a single .json file which is huuuuge as soon as you add two or three tests.

We should find a way to organize these tests in separated files. As tests allow (or shall allow) running multiple tests for one operator, we should think in a way to sort them properly.

A part from splitting the .json file into independent files it may be very interesting to use yaml files instead of json, as they give us advantages like yaml templates which will allow us to reduce our operator test files.

refs #24

[elboletaire]

Closed in 8526f86