chg: test: Prioritize the 10 slowest system test scopes

Update PRIORITY_TESTS with the 10 longest-running test
scopes measured from CI (job 7468217).  These get scheduled
first so that with --dist=loadscope they land on separate
workers instead of piling up at the end.

Also fix "serve-stale/" to "serve_stale/" to match the
actual directory name, and add a startup check that fails
if any PRIORITY_TESTS entry does not match an existing
directory.

Assisted-by: Claude:claude-opus-4-7
Merge branch 'mnowak/prioritize-slow-system-tests' into 'main'

See merge request isc-projects/bind9!12104