Fix need/checks for inputdata path in subset_data and Python testing

[samsrabin]

The following Python unit tests fail on non-NCAR machines with inputdata directory does not exist:

• test_check_args_outsurfdat_provided
• test_inputdata_setup_files_basic

That error is being thrown in setup_files() here:

CTSM/python/ctsm/subset_data.py

Lines 564 to 571 in bfc5c31

	if args.inputdatadir == "defaults.cfg":
	clmforcingindir = defaults.get("main", "clmforcingindir")
	else:
	clmforcingindir = args.inputdatadir
	if not os.path.isdir(clmforcingindir):
	logger.info("clmforcingindir does not exist: %s", clmforcingindir)
	abort("inputdata directory does not exist")

However, those tests don't really need that directory at all.

• test_check_args_outsurfdat_provided doesn't need the inputdata dir at all. setup_files() should be rewritten so that check is optional.
• test_inputdata_setup_files_basic checks that the inputdata dir is /glade/campaign/cesm/cesmdata/cseg/inputdata, which is just hard-coding things so that this test will fail outside Derecho and Casper. Why? I also don't really get the point of the other two assertions there.