Nix: development workflow differs from CI

[brianhenn]

Noting a couple things in the nix development workflow as described in the README that differ from CI. Also noting that some of these things don't seem to be invoked anywhere on CI and therefore presumably are broken.

• nix readme indicates to build the model with cd FV3 && ./configure nix and make build_native (which is $(MAKE) -j 8 -C FV3 GCOV=Y), whereas on CI it is built with cd FV3 && bash configure nix && make -j 4. In other words, GCOV is invoked in the readme description but not on CI. The wrapper tests then fail on import when GCOV is invoked in the model build. Not clear to me where GCOV is actually used.

• nix readme indicates that wrapper tests are invoked with make test_native which calls pytest FV3/wrapper/tests/. However on CI the wrapper tests are invoked with make test_wrapper, which calls pytest -v FV3/wrapper/tests/pytest and pytest -v FV3/wrapper/tests/test_all_mpi_requiring.py. In other words, the readme set of tests is broader than the CI tests, and many of the extra tests (the other ones in FV3/wrapper/tests) included in the readme description do not pass in the local nix environment. Not clear to me under what context those tests pass, if ever.