Crate filter tests

[gramanas]

Following PR #1263 here are four five tests for the crate filters.
The code is commented and what is happening should be pretty self explanatory.

[Be-ing]

Looks good so far. I thought the tests would have to clean up the database at the end of each test adding to the DB, but ~MixxxTest deletes all the temporary test files between each test run, so that isn't necessary.

It would be good to add a test combining crate filters like "crate:testCrate1 crate:testCrate2" and "crate:testCrate1 -crate:testCrate2". You can put these in the same test so you don't have to duplicate setting up the test crates and adding the test tracks. Add two tracks to each crate, with one track being in common in both crates.

[Be-ing]

LGTM. @uklotzde, any comments?

[uklotzde]

LGTM. It would be possible to reduce the amount of redundant code among the very similar setup procedures for different tests. Just a minor remark, nothing that prevents merging this PR.

Thanks for completing the new feature with appropriate tests, Anast!

[daschuer]

Thank you all working on this!