test-submit-model.R: improve .mode override test

The test that passing .mode overrides the bbr.bbi_exe_mode option
doesn't account for bbr.bbi_exe_mode being "local" on non-Linux
machines (i.e. the test would pass even if the argument didn't have an
effect).

Use the same approach that other tests do to explicitly choose a mode
other than the default.  Also extract the check and put it in its own
test next to a similar check.

tests/testthat/test-submit-model.R

@@ -26,12 +26,6 @@ test_that("submit_model(.dry_run=TRUE) returns correct command string", {
     as.character(glue("{cmd_prefix} {default_mode} {mod_ctl_path} --overwrite --threads=4 --parallel"))
   )
 
-  # switch to local mode
-  expect_identical(
-    submit_model(MOD1, .mode = "local", .dry_run = TRUE)[[PROC_CALL]],
-    as.character(glue("{cmd_prefix} local {mod_ctl_path} --overwrite --threads=4 --parallel"))
-  )
-
   # over-riding yaml arg with passed args
   expect_identical(
     submit_model(MOD1,
@@ -86,6 +80,14 @@ test_that("submit_model() throws an error if passed `output_dir` bbi arg", {
   )
 })
 
+test_that("submit_model() .mode argument overrides bbr.bbi_exe_mode default", {
+  other_mode <- if (identical(default_mode, "local")) "slurm" else "local"
+  expect_identical(
+    submit_model(MOD1, .mode = other_mode, .dry_run = TRUE)[[PROC_CALL]],
+    as.character(glue("{cmd_prefix} {other_mode} {mod_ctl_path} --overwrite --threads=4 --parallel"))
+  )
+})
+
 test_that("submit_model(.mode) inherits option", {
   other_mode <- if (identical(default_mode, "local")) "slurm" else "local"
   withr::with_options(list(bbr.bbi_exe_mode = other_mode), {