test: Only build osx tests for OS_MACOS

Fixes CMake build error on Linux:

CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
COCOA (ADVANCED)
    linked by target "macOS_test" in directory /var/tmp/paludis/build/media-video-obs-studio-28.0.0-beta1/work/obs-studio-28.0.0-beta1/test/osx

test/CMakeLists.txt

@@ -5,7 +5,7 @@ if(BUILD_TESTS)
     add_subdirectory(win)
   endif()
 
-  if(OS_POSIX)
+  if(OS_MACOS)
     add_subdirectory(osx)
   endif()
 endif()