Add basic gripper controller tests

[bmagyar]

I've added some basic lifecycle-related tests to gripper controllers
Putting up for PR as-is but happy to add the action interface tests too

[codecov-commenter]

Codecov Report

Merging #459 (2d22d16) into master (e7f9962) will decrease coverage by 6.03%.
The diff coverage is 20.21%.

@@            Coverage Diff             @@
##           master     #459      +/-   ##
==========================================
- Coverage   35.78%   29.74%   -6.04%     
==========================================
  Files         189        7     -182     
  Lines       17570      743   -16827     
  Branches    11592      428   -11164     
==========================================
- Hits         6287      221    -6066     
+ Misses        994      162     -832     
+ Partials    10289      360    -9929     

Flag	Coverage Δ
unittests	29.74% <20.21%> (-6.04%)	⬇️

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Impacted Files	Coverage Δ
...de/diff_drive_controller/diff_drive_controller.hpp	100.00% <ø> (ø)
...ontroller/test/test_load_diff_drive_controller.cpp	12.50% <0.00%> (ø)
diff_drive_controller/src/odometry.cpp	42.16% <11.11%> (ø)
...ive_controller/test/test_diff_drive_controller.cpp	17.62% <12.08%> (ø)
diff_drive_controller/src/speed_limiter.cpp	46.55% <13.33%> (ø)
...troller/include/diff_drive_controller/odometry.hpp	20.00% <20.00%> (ø)
...iff_drive_controller/src/diff_drive_controller.cpp	32.13% <24.11%> (ø)
...dcaster/test/test_load_joint_state_broadcaster.cpp
...ler/test/test_load_joint_trajectory_controller.cpp
...ory_controller/src/joint_trajectory_controller.cpp
... and 192 more