Add missing test_depend ament_cmake_gmock

[sloretz]

Fixes failure to build Rolling CI introduced in #636

https://build.ros2.org/job/Rci__nightly-connext_ubuntu_focal_amd64/221/console
https://build.ros2.org/job/Rci__nightly-cyclonedds_ubuntu_focal_amd64/225/console

03:28:18 CMake Error at CMakeLists.txt:64 (find_package):
03:28:18   By not providing "Findament_cmake_gmock.cmake" in CMAKE_MODULE_PATH this
03:28:18   project has asked CMake to find a package configuration file provided by
03:28:18   "ament_cmake_gmock", but CMake did not find one.
03:28:18 
03:28:18   Could not find a package configuration file provided by "ament_cmake_gmock"
03:28:18   with any of the following names:
03:28:18 
03:28:18     ament_cmake_gmockConfig.cmake
03:28:18     ament_cmake_gmock-config.cmake
03:28:18 
03:28:18   Add the installation prefix of "ament_cmake_gmock" to CMAKE_PREFIX_PATH or
03:28:18   set "ament_cmake_gmock_DIR" to a directory containing one of the above
03:28:18   files.  If "ament_cmake_gmock" provides a separate development package or
03:28:18   SDK, be sure it has been installed.
```

[sloretz]

CI (build: --packages-up-to rosbag2_compression_zstd test: --packages-select rosbag2_compression_zstd)

• Linux
• Linux-aarch64
• macOS
• Windows

[audrow]

Looks good, with green CI (I see it's almost done)!

[emersonknapp]

Thanks for the quick fix! This one's my bad!