We never ever want anything OpenCL-related in the sc_ucalc unit test

So unset any SC_BACKGROUND_COMPILATION environment variable to avoid that
mixing up things when running this test. (It doesn't in a "clean" build, but
in a build where you temporarily have made the SC_BACKGROUND_COMPILATION thing
actually work in the sc_opencl_test unit test (by bypassing the weird "Is
OpenCL enabled? Yes! No! Yes!" dance there), it does.)

Loading the scopencl library will load also the sc library, which causes lots
of confusion, as this unit test library itself includes all the object files
from the sc library. (For IMHO questionable micro-optimisation reasons.)

Another day wasted thanks to not understanding what undoubtedly are clever and
elegant tricks.

Change-Id: I203b6dae7d9b52787ac2d57180be87a61626af6c