Fix gmock CMP0115 (backport #1830)

[mergify]

Fixes

--- stderr: controller_manager                                           
CMake Warning (dev) at /opt/ros/rolling/share/ament_cmake_gmock/cmake/ament_add_gmock_executable.cmake:50 (add_executable):
  Policy CMP0115 is not set: Source file extensions must be explicit.  Run
  "cmake --help-policy CMP0115" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  File:

    /workspaces/ros2_rolling_ws/src/ros2_control/controller_manager/test/test_hardware_spawner.cpp
Call Stack (most recent call first):
  /opt/ros/rolling/share/ament_cmake_gmock/cmake/ament_add_gmock_executable.cmake:37 (_ament_add_gmock_executable)
  /opt/ros/rolling/share/ament_cmake_gmock/cmake/ament_add_gmock.cmake:65 (ament_add_gmock_executable)
  CMakeLists.txt:197 (ament_add_gmock)
This warning is for project developers.  Use -Wno-dev to suppress it.

---

This is an automatic backport of pull request #1830 done by [Mergify](https://mergify.com).

[christophfroehlich]

needs #1828 first

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.93%. Comparing base (a76101d) to head (f7304f3).
Report is 1 commits behind head on iron.

Additional details and impacted files

@@            Coverage Diff             @@
##             iron    #1849      +/-   ##
==========================================
- Coverage   62.96%   62.93%   -0.03%     
==========================================
  Files         113      113              
  Lines       13254    13254              
  Branches     8968     8965       -3     
==========================================
- Hits         8345     8342       -3     
  Misses        898      898              
- Partials     4011     4014       +3     

Flag	Coverage Δ
unittests	62.93% <ø> (-0.03%)	⬇️

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see 2 files with indirect coverage changes