Convert test infrastructure targets to static library targets.

Convert test infrastructure targets to static library targets.

Convert CMake function `gmx_add_unit_test_object_library()` to
`gmx_add_unit_test_library()` and produce STATIC instead of OBJECT
targets. Update `mdrun_test_objlib` to `mdrun_test_infrastructure`
using the helper function.

This allows the target to use `target_link_libraries` to get the
compiler flags, definitions, and include directories necessary for
compatibility with `gmock` and other resources. Resolves some noisiness
from googletest headers under the standard GROMACS `-Wundef` flag.
Relevant to related change that migrate from global GMOCK variables to
modern usage of gtest and gmock CMake targets.

Removed unnecessary TESTUTILS_LIBS variable.

Refs #3033

Change-Id: Id5b47774b679e8e205f899ea80df65153da44516