run_sys_tests: Check Python environment for FatesColdTwoStream tests

[samsrabin]

Description of changes

FatesColdTwoStream and FatesColdTwoStreamNoCompFixedBioGeo tests use the $FATESDIR/tools/modify_fates_paramfile.py script, which requires certain non-standard Python packages. Now, run_sys_tests will check for the availability of those packages.

Specific notes

Uses existing _try_systemtests() function (now generalized and renamed _check_py_env()).

Contributors other than yourself, if any: None

CTSM Issues Fixed (include github issue #): n/a

Are answers expected to change (and if so in what way)? No.

Any User Interface Changes (namelist or namelist defaults changes)? No.

Testing performed, if any:
Unloaded all modules on Izumi and tried to submit run_sys_tests using -s, -f, and -t arguments that requested those tests. It successfully errors, saying:

Traceback (most recent call last):
  File "/home/samrabin/ctsm_scale-mat-reqs/python/ctsm/run_sys_tests.py", line 731, in _check_py_env
    import tools.modify_fates_paramfile
  File "/home/samrabin/ctsm_scale-mat-reqs/python/ctsm/../../src/fates/tools/modify_fates_paramfile.py", line 26, in <module>
    import numpy as np
ModuleNotFoundError: No module named 'numpy'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "./run_sys_tests", line 20, in <module>
    main(cime_path=cime_path)
  File "/home/samrabin/ctsm_scale-mat-reqs/python/ctsm/run_sys_tests.py", line 93, in main
    extra_create_test_args=args.extra_create_test_args,
  File "/home/samrabin/ctsm_scale-mat-reqs/python/ctsm/run_sys_tests.py", line 252, in run_sys_tests
    _check_py_env(testname_list)
  File "/home/samrabin/ctsm_scale-mat-reqs/python/ctsm/run_sys_tests.py", line 733, in _check_py_env
    raise ModuleNotFoundError("modify_fates_paramfile" + err_msg) from err
ModuleNotFoundError: modify_fates_paramfile can't be loaded. Do you need to activate the ctsm_pylib conda environment?

It also works on Derecho.

black and pylint pass.

[samsrabin]

@ekluzek Done!