Add test for successful_ignitions mode for FATES-SPITFIRE

Per @ekluzek [comment](https://github.com/ESCOMP/CTSM/pull/1264#discussion_r566406680), we should create a testmod option for enabling `fates_spitfire_mode = 3`. Currently this is only referenced in `Fates_nat_and_anthro_ignitions` testmod (which itself enables `fates_spitfire_mode = 4`):

https://github.com/ESCOMP/CTSM/blob/5f348cac1919e70df19a6edbbf7b970c3877ce22/cime_config/testdefs/testmods_dirs/clm/Fates_nat_and_anthro_ignitions/README#L14-L26

The above should be implemented in a new test and the data brought to CESM data (if possible). We should strike this language from `Fates_nat_and_anthro_ignitions` and update the name of `Fates_nat_and_anthro_ignitions` to be more consistent with other test name formats and to reflect the fact that it only tests anthropogenic ignitions.