[Serve] [CI] Split test_serve_dashboard into two test suites

[shrekris-anyscale]

Why are these changes needed?

test_serve_dashboard.py has been flaky recently due to timeouts. This change splits test_serve_dashboard.py into two test suites.

Once #40048 is completed, we can stop running tests on both the dashboard agent and the head, which should help reduce the test time.

Related issue number

Addresses #46459.

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • This change updates test_serve_dashboard.