Add a test template for plugin implementations

[christophfroehlich]

The test cases are independent of the implementation, and all implementations should succeed with the tests.

I haven't touched the testcases itself, only moved it to a header file which is installed as INTERFACE library if BUILD_TESTING is true.
Maybe we should install it always, so that the header file is shipped with binaries from the buildfarm and users of custom plugins could use it?

[mergify]

This pull request is in conflict. Could you fix it @christophfroehlich?

[mergify]

This pull request is in conflict. Could you fix it @christophfroehlich?

[mergify]

This pull request is in conflict. Could you fix it @christophfroehlich?

[mergify]

This pull request is in conflict. Could you fix it @christophfroehlich?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.62%. Comparing base (7a77940) to head (b5a1e93).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
- Coverage   88.11%   87.62%   -0.49%     
==========================================
  Files           4        5       +1     
  Lines         303      291      -12     
  Branches       53       53              
==========================================
- Hits          267      255      -12     
  Misses         21       21              
  Partials       15       15              

Flag	Coverage Δ
unittests	87.62% <100.00%> (-0.49%)	⬇️

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Files with missing lines	Coverage Δ
...terface/test/kinematics_interface_common_tests.hpp	100.00% <100.00%> (ø)
...terface_kdl/test/test_kinematics_interface_kdl.cpp	100.00% <100.00%> (ø)

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