Simplify compiling GPU code for tests

Simplify compiling GPU code for tests

Documented and extended gmx_add_gtest_executable to let us annotate
which source files may need special treatment for GPU
compilation. Those details are all now hidden in the function rather
than complicating the CMakeLists.txt files in multiple places. There's
a lot of incidental lines changed to make sure people can use the
rearrangement well, but the benefit is illustrated in the following
files:

gpu_utils/test/CMakeLists.txt
mdlib/test/CMakeLists.txt

Accordingly, various test code no longer compiles into
libgromacs. Removed some completely unnecessary compilation of CUDA
code for tests.

Compiling test .cu files that also need MPI libraries is not currently
supported. Thus, removed am unnecessary header dependency in
mdlib/tests on MPI libraries that was not needed (and awkward for nvcc
compilation).

Found and fixed a bug where
src/gromacs/gpu_utils/tests/typecasts_runner.cpp wasn't compiled when
it was intended to be.

Change-Id: I9ba624094092a9951fa63f3264c9d99d7ba0cbd1