(main) ccpp_prebuild: fix hardcoded name of ccpp_t host variable, update CCPP stub, fix capgen unit tests

[climbfuji]

Turns out the CCPP stub in directory stub is currently broken in main, following an earlier PR.

This PR fixes the problem in the ccpp_prebuild.py code by removing the hardcoded name of the CCPP data (type ccpp_t variable and replacing it with the actual name from the host model that gets passed to the group caps through the API and suite cap.

I took this opportunity to replace the ugly in-source build of the CCPP stub with an out-of-source build.

And while testing I discovered that the capgen unit tests are also broken ... simple fix in this PR included.

Note that each part of this PR is a separate commit so that individual parts can easily be identified cherry-picked, if necessary (thus this PR should be a merge commit, not a squash merge).

User interface changes?: No

Fixes: https://github.com/NCAR/ccpp-framework/issues/496 Fixes https://github.com/NCAR/ccpp-framework/issues/497

Testing: test removed: 0 unit tests:

• ccpp_prebuild tests in tests pass
• capgen tests in test pass system tests:
• none conducted - someone needs to test this with the ufs-weather-model and the ccpp-scm (I expect no impact) @dustinswales @grantfirl @mkavulich is this something you can help with? manual testing:
• ccpp-framework stub instructions work

[climbfuji]

thanks for the quick turnaround!