No testdata copy and circular symlink fix

[daschuer]

This PR removes the mixxx-testdata target that copies/symlinks the test data into the build directory.
Instead it makes use of the existing function that is looking up the source tree path form CMakeCache.
This replaces the hard coded paths in the test classes.

This also fixes a circular symlink that happens when changing from 2.3 to main reported here:
https://mixxx.zulipchat.com/#narrow/stream/247620-development-help/topic/Failing.20controller.20test

[JoergAtGithub]

I tried this PR locally on Win7 VS2019 and it works flawless. But I got warnings at compile time:

  [265/537] Building CXX object CMakeFiles\mixxx-lib.dir\src\preferences\configobject.cpp.obj
C:\Users\Joerg\source\repos\JoergAtGithub\mixxx\src\preferences\configobject.cpp(329): warning C4661: 'QString ConfigObject<ConfigValueKbd>::computeResourcePath(void)': no suitable definition provided for explicit template instantiation request
  C:\Users\Joerg\source\repos\JoergAtGithub\mixxx\src\preferences/configobject.h(180): note: see declaration of 'ConfigObject<ConfigValueKbd>::computeResourcePath'
C:\Users\Joerg\source\repos\JoergAtGithub\mixxx\src\preferences\configobject.cpp(300): warning C4661: 'QString ConfigObject<ConfigValueKbd>::computeResourcePath(void)': no suitable definition provided for explicit template instantiation request
  C:\Users\Joerg\source\repos\JoergAtGithub\mixxx\src\preferences/configobject.h(180): note: see declaration of 'ConfigObject<ConfigValueKbd>::computeResourcePath'

[daschuer]

Please retry, the warning should be gone.

[JoergAtGithub]

I confirm, the warning is gone!

[ronso0]

I'm going to ng through this commit by commit but it'll tkae some time since I'm nit familiar with the tests.

[daschuer]

Done.

[uklotzde]

The type-safe return value improved the code considerably.

Please fix the remaining redundancy. My proposal was just a first idea.

[daschuer]

Done

[uklotzde]

Thank you for simplifying the build and code! LGTM