Tests: bundle tests for some modules in their own executables

The ffmpeg, guardedalloc and blenlib are quite isolated and putting them in
their own executable separate from blender_test is faster for development than
linking the entire blender_tests executable.

For Cycles, this also bundles all the unit tests into one executable.

I also considered going back to having every test file in its own executable,
but the size of these executables does add up quickly, and linking and running
all blenlib tests is still < 1s here.

Ref T79958

Reviewed By: LazyDodo

Maniphest Tasks: T79958

Differential Revision: https://developer.blender.org/D8714

CMake: clean up setting of platform specific linker flags

Set flags directly on the target, and use common function for all cases.