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/user/CMakeLists.txt

@@ -12,11 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-mujoco_test(user_model_test)
-target_link_libraries(user_model_test fixture gmock absl::str_format)
+mujoco_test(user_model_test ADDITIONAL_LINK_LIBRARIES absl::str_format)
 
 mujoco_test(user_objects_test)
-target_link_libraries(user_objects_test fixture gmock)
 
 mujoco_test(
   user_api_test
@@ -24,24 +22,13 @@ mujoco_test(
   ENVIRONMENT
   "MUJOCO_PLUGIN_DIR=$<TARGET_FILE_DIR:elasticity>"
 )
-target_link_libraries(user_api_test fixture gmock)
 
 mujoco_test(user_flex_test)
-target_link_libraries(user_flex_test fixture gmock)
 
-mujoco_test(user_mesh_test)
-target_link_libraries(
-  user_mesh_test
-  fixture
-  gmock
-  absl::str_format
-)
+mujoco_test(user_mesh_test ADDITIONAL_LINK_LIBRARIES absl::str_format)
 
 mujoco_test(user_composite_test)
-target_link_libraries(user_composite_test fixture gmock)
 
 mujoco_test(user_resource_test)
-target_link_libraries(user_resource_test fixture gmock)
 
 mujoco_test(user_vfs_test)
-target_link_libraries(user_vfs_test fixture gmock)