chg: test: Refactor configloading test

- Use WatchLog.wait_for_sequence() for the configloading test.
- Omit artifacts check, as it seems quite useless for this test case.
- Join all the tests together. The test case is fairly simple here and
  this is the easiest way to ensure the log will be in a predictable
  state for all tests. Previously, there was no way to ensure
  test_configloading_loading() won't be executed after the other tests,
  which would render the check moot. It could also be separated into
  its own module, but that seems excessive for a simple test case like
  this.
- Use jinja2 template for named.conf and remove setup.sh.
- Remove README and put the relevent comment directly next to the test.
- Remove _sh_ from the test filename to uphold the naming convention.

Merge branch 'nicki/refactor-configloading-test' into 'main'

See merge request isc-projects/bind9!10748