Make it possible to run tests. (#2553)
Make it possible to run tests. (#2553)

I followed the instructions on
https://github.com/polybar/polybar/wiki/Testing, and this happened to
me:

    $ cmake .. -DBUILD_TESTS=ON
    ...
    [ 11%] Creating directories for 'googletest'
    [ 22%] Performing download step (git clone) for 'googletest'
    Cloning into 'googletest-src'...
    fatal: invalid reference: master
    CMake Error at googletest-download/googletest-prefix/tmp/googletest-gitclone.cmake:40 (message):
      Failed to checkout tag: 'master'

    make[2]: *** [CMakeFiles/googletest.dir/build.make:99: googletest-prefix/src/googletest-stamp/googletest-download] Error 1
    make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/googletest.dir/all] Error 2
    make: *** [Makefile:91: all] Error 2
    CMake Error at tests/CMakeLists.txt:19 (message):
      Build step for googletest failed: 2

It looks like the problem is that
[googletest](https://github.com/google/googletest/) changed their branch from
`master` to `main` at some point.

I also fixed a broken link
https://github.com/polybar/polybar/wiki/Testing. Hopefully it's
intentional that random members of the public such as myself are allowed
to do that?
jaagr/polybar,jaagr/polybar,jaagr/polybar