googletest (gtest) is more feature rich than the current implementation
which only provides expect() which is basically an assertion. It is also
quite intuitive to use, this can be seen in the rewrite of the
command_line test where EXPECT_THROW replaces a whole try-catch block.
I have also moved the source files the test depend on to be linked in
CMakeLists.txt instead of including them directly because include .cpp
files is bad.
The two x11 tests were removed because they were written two years ago
and a lot of the things they depend on, don't actually exist anymore in
polybar (I think we switched to xpp after those tests were written)
Tests are now compiled with the gcov lib which can be used to provide
test coverage in a second step
