Execute tests in binary dir

[xeruf]

Prevents the creation of the test files, which were accidentally committed in 0cd2aec, in the project root. Instead use the binary dir.

See https://mixxx.zulipchat.com/#narrow/stream/109171-development/topic/Running.20tests.20with.20ctest.20creates.20untracked.20files/near/203089624

[xeruf]

I am currently testing whether we can get rid of manually specifying the working directory completely.

[xeruf]

Yep, works. We can just rely on defaults :)

[xeruf]

Would be great if we could merge this soon, before these extraneous files are merged into everything.
Also, should I maybe point it to one of the stable branches (2.3) to also have it there?

[Holzhaus]

LGTM, thank you!

[uklotzde]

AppVeyor Windows builds on master seem to be failing since this PR has been merged!?

The test data files are not available in cmake_build directory:

TagLib - Cannot read audio properties from inaccessible/unreadable/invalid file: 
MetadataSourceTagLib - No track metadata or cover art found in file "C:/projects/mixxx/cmake_build/src/test/id3-test-data/TOAL_TPE2.mp3" with type 3
..\src\test\trackupdate_test.cpp(93): error: Value of: pTrack->isMetadataSynchronized()
  Actual: false
Expected: true

[uklotzde]

This needs to be fixed urgently!

[xeruf]

Oof, this is weird. Why does it work on Linux?