Make tests work with CTest

[JL2210]

tests expect rgb{asm,link,fix,gfx} to be in ./

add CTestCustom.cmake to pull in dependencies

add option to disable non-free tests

Switch find_package conditions to explicitly state that it's broken on MacOS, and use pkg-config there only.

[ISSOtm]

Looks about fine to me. Since in the end we are still using pkg-config for libpng, I think #1543 can also be closed?

[JL2210]

Looks about fine to me. Since in the end we are still using pkg-config for libpng, I think #1543 can also be closed?

Yeah. FindPNG is pretty unstable, evidently.

[Rangi42]

Note that these are run by doing cmake --build build --target test (after the usual cmake --build build -j4 --verbose and cmake --install build --verbose).