Add fixtures for pool core tests and controller plugin tests

I suggest to review this MR in the following order.

1. Documentation
   • Developer's Guide -> Writing controllers -> How to test controller plugins
   • Developer's Guide -> API -> Library -> sardana.pool.test
2. Code review of fixtures and utilities added to sardana.pool.test.util.
3. Code review of tests added to sardana.pool.test sub-modules.
4. See exemplary usage of test fixtures in the sardana-tango controller plugin tests.

For running the test locally you will need to merge first the Merge request dependencies.

@sardana-org, I assign the reviewer to @cpt-majkel since we started this development togehter but feedback from all of you is very important here. I hope that the fixtures provided in this MR will make the pool core development and the controller plugin development easier an more maintainable.

Note, it still misses the fixture for the data acquisition part.