Blender Git Loki

Fix: Unit tests on windows.

Problem was twofold

1) `GENERATOR_IS_MULTI_CONFIG` is a property not a variable so
the test for it would always be false, unless you set a custom
CMAKE_INSTALL_PREFIX (like the buildbot does) the unit tests
would have a wrong working directory and complain about missing
dlls or blender executable

2) Tests added outside of `/test` (like libmv) would have no working
folder set since the variable would not be visible for them.

consulted @sergey who voiced the opinion that duplicating the code
to the test macro was slightly less evil than moving it to the main
CMakeLists.txt