Add BUILD_TESTS Flag to Readme for CMake [skip ci]

[wienans]

See title as i was developing on Linux didn't found the option for building the test binary. With some feedback and examination of the Actions i found that the BUILD_TESTS flag should be enabled. Added that to Readme.

[firewave]

Thanks for your contribution.

I wanted to comment that this does not need to be documented as it is a regular CMake option but that is not correct.

The option I was thinking of if actually BUILD_TESTING: https://cmake.org/cmake/help/latest/module/CTest.html. We should deprecate the existing one and move to the common one.

As there are many more options you could set the documentation should point to the option() entries in cmake/options.cmake. Unfortunately there appears to be no way to generate a list of those. Calling cmake -LAH also includes all cached variables which makes things more complicated.