Clean up field unit tests

[wojcikstefan]

@ephraimberkovitch fyi you pointed out in #1485 (comment) that many tests follow an invalid pattern. This PR is an attempt at cleaning some of it up. Feel free to submit similar patches for other parts of the codebase!

High level summary of the changes:

• Removed some redundant whitespace.
• Removed all redundant drop_collection calls (we only need those in the beginning of a test).
• Improved clarity of some docstrings.
• Tweaked some of the methods to test the same behavior in a more efficient manner.
• Minor PEP8 fixes.
• Break tests pertaining to the CachedReferenceField into a separate test class (makes the grouping cleaner and also prepares the code nicely before a larger inspection - many users have complained that the CachedReferenceField is buggy and in many cases unusable and might even be considered for removal).

[ephraimberkovitch]

@wojcikstefan, I will clean up more tests soon. Another question - do you have any critical bugs to be resolved now, which you would like me to deal with?