Fix: add test case for CanMsgRingBuffer.

[jboynes]

Fixes a deprecated use of volatile in CanMsgRingbuffer.
Also added a basic testcase to make sure the CPP file is compiled/tested.

Relies on PR #206 to reliably run the tests under C++20

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.09% ⚠️

Comparison is base (ebe1fab) 95.61% compared to head (1765628) 95.52%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
- Coverage   95.61%   95.52%   -0.09%     
==========================================
  Files          14       16       +2     
  Lines        1048     1072      +24     
==========================================
+ Hits         1002     1024      +22     
- Misses         46       48       +2     

Files Changed	Coverage Δ
api/CanMsgRingbuffer.cpp	89.47% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[aentinger]

I don't see any change pertaining volatile (unless you refer to the change of unary increment/decrement operators to a binary arithmetic operation), but I do appreciate adding the test case.