Re-add cpp coverage

[ghost]

• Add -DCOVERAGE=True option to cpp CMakeLists.txt
• Also re-add macOS test with allowing for failure

[codecov]

Codecov Report

Merging #705 into master will increase coverage by 0.69%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #705      +/-   ##
=========================================
+ Coverage    73.9%   74.6%   +0.69%     
=========================================
  Files         104     162      +58     
  Lines       10446   14804    +4358     
  Branches     1431    1431              
=========================================
+ Hits         7720   11044    +3324     
- Misses       2421    3455    +1034     
  Partials      305     305

Impacted Files	Coverage Δ
sdk/cpp/core/src/entity_util.cpp	75% <0%> (ø)
sdk/cpp/core/src/ydk.cpp	25.52% <0%> (ø)
sdk/cpp/core/src/executor_service.cpp	90.38% <0%> (ø)
sdk/cpp/core/src/restconf_provider.cpp	100% <0%> (ø)
sdk/cpp/core/src/codec_service.cpp	73.43% <0%> (ø)
sdk/cpp/core/src/logging_callback.hpp	0% <0%> (ø)
sdk/cpp/core/tests/core_test.cpp	100% <0%> (ø)
sdk/cpp/core/src/path/restconf_session.cpp	89.58% <0%> (ø)
sdk/cpp/core/src/opendaylight_parser.cpp	95.38% <0%> (ø)
sdk/cpp/core/src/bits.cpp	100% <0%> (ø)
... and 48 more

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