Add support for setting arbitrary additional testmods via create_test

Given the way things are split among CIME, CMEPS, and ccs_config, this might not be reasonable to implement... but it would be great if I could set `JOB_PRIORITY` via command line argument when running `create_test`, with bonus points for also being able to add it to the `[create_test]` block of `${HOME}/.cime/config`

(Edit by billsacks 2026-09-02: This is growing into a more general desired functionality: see https://github.com/ESMCI/cime/issues/5013#issuecomment-5051983291. So I'm changing the title from "Add support for setting JOB_PRIORITY via create_test" to "Add support for setting arbitrary additional testmods via create_test".)

Start by tracing the create_test command and how the [create_test] block in ${HOME}/.cime/config is processed across CIME, CMEPS, and ccs_config. Define how arbitrary additional testmods, including JOB_PRIORITY, should be supplied from the command line and configuration, then verify that both paths apply them consistently.

Written by the indexing model from the issue text.