Move some FATES default filenames to namelist_defaults

There are some default filenames for FATES that are preserved in the testmod rather than in namelist_defaults. Keeping them in namelist_defaults makes it easier for users to NOT have to know the filenames themselves -- namelist defaults just gives it for you. This makes it easier to run these cases for users and more robust as well, with also removing some of the specific changes needed for the tests themselves.

This was noticed in ctsm5.3.014 as I removed hardcoded checks for existence of these files in build-namelist. By putting them into add_defaults it will already check for existence so there isn't special code needed.

The filenames I see that need this are:
- flandusepftdat
- fluh_timeseries

There's also fates_inventory_ctrl_filename, but we currently don't have defaults for it that we test yet.

The conversation that started this is:

I removed the check for file existence here, because it was too early and caused problems. The check for file existence is a general thing that should be done for all filenames anyway, so doesn't need to have special code to handle it for each one here.

However, this now is still failing to check for file existance at the right time, which causes a build-namelist test to fail. I'm looking into this now...

_Originally posted by @ekluzek in https://github.com/ESCOMP/CTSM/pull/2888#discussion_r1868458463_

The error that's seen in the namelist unit tester is:

```
1237/3263 < FAIL>
```