Reduce cmake test boilerplate

All instances of mujoco_test in the code base were followed by
a call to  target_link_libraries(<testname> fixture gmock).

As anyhow mujoco_test already was calling target_link_libraries
to some predefined list of targets (mujoco and gtest_main),
this PR adds to the list of default linked targets also fixture
and gmock, to reduce the boilerplate.

Furthermore, to completly remove the need for calling
target_link_libraries after a call to mujoco_test, this PR also
add to the mujoco_test macro the ADDITIONAL_LINK_LIBRARIES
argument, that can be used if a given test needs to link some
additional targets beside the default ones.

To permit to use these new features also for mujoco benchmarks,
this PR adds a MAIN_TARGET parameter to mujoco_test, to select
if gtest_main or another target is used to provide the main
entry point to the test executable.

test/plugin/sensor/CMakeLists.txt

@@ -18,4 +18,3 @@ mujoco_test(
   ENVIRONMENT
   "MUJOCO_PLUGIN_DIR=$<TARGET_FILE_DIR:sensor>"
 )
-target_link_libraries(sensor_test fixture gmock)