For multi-config generators, tests are in `bin/tests/<config>` folder.
and OpenMP cannot be found at `@executable_path/../Resources/lib`.
So create that folder and put the library there to be used by tests.
It is not ideal to copy the library around. When minimum CMake version
is changed to 3.12, FindOpenMP by CMake can be used for this whole
block of code. Then a nice rpath based solution can be used.