Improve robustness of transmission tests

[bmagyar]

Background

Improve the robustness of transmission tests by making sure all floating-point value comparisons are using an approximate equality check.

In practice, this means that with Google Mock we use ASSERT_NEAR or DoubleNear instead of ASSERT_EQ or ==. Check that the tests inside transmission_interface/test follow this. The EPS value used for these tests could perhaps be relaxed slightly too.

Here's an example error we see on the CI from time-to-time:

 build/transmission_interface/test_results/transmission_interface/test_four_bar_linkage_transmission.gtest.xml: 8 tests, 0 errors, 1 failure, 0 skipped
- transmission_interface.BlackBoxTest IdentityMap
  <<< failure message
    /home/runner/work/ros2_control/ros2_control/.work/target_ws/src/ros2_control/transmission_interface/test/four_bar_linkage_transmission_test.cpp:174
    Value of: ref_val[1]
    Expected: is approximately 508.18968922836888 (absolute error <= 9.9999999999999995e-07)
      Actual: 508.19 (of type double), which is -1.0207e-06 from 508.19
    /home/runner/work/ros2_control/ros2_control/.work/target_ws/src/ros2_control/transmission_interface/test/four_bar_linkage_transmission_test.cpp:174
    Value of: ref_val[1]
    Expected: is approximately 177.99605718539684 (absolute error <= 9.9999999999999995e-07)
      Actual: 177.996 (of type double), which is 1.38017e-06 from 177.996

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[taDachs]

I would like to pick up this issue.

[christophfroehlich]

I think this was solved by #1238? Thanks @taDachs