[9.18] fix: test: ISC_RUN_TEST_IMPL should use a static declaration

These functions don't need to be called from multiple places and
by making them static we will detect when they are not added to the
list functions to be tested.

Closes #5715

Backport of MR !11480

Merge branch 'backport-5715-isc_run_test_impl-should-use-a-static-declaration-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11503