GHA: Add CMake subdir test

[Flamefire]

See https://github.com/boostorg/boost-ci/blob/master/CMake.md#ci-tests for a description

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.78%. Comparing base (d4be7a4) to head (bc0d2ba).
Report is 16 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #82      +/-   ##
===========================================
- Coverage    95.80%   95.78%   -0.03%     
===========================================
  Files            5        5              
  Lines          668      664       -4     
===========================================
- Hits           640      636       -4     
  Misses          28       28              

see 1 file with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8ad7522...bc0d2ba. Read the comment docs.