Add async regression test for api_key_prefix alias fallback

Mirror the sync test_auth_settings_with_authorization_key_and_prefix
added in this PR, covering the same path in the async variant of
get_api_key_with_prefix() via auth_settings().

Also fixes InClusterConfigTest base class from unittest.TestCase to
IsolatedAsyncioTestCase so the existing async test_refresh_token
actually runs.

Signed-off-by: HaimLC <110099998+HaimLC@users.noreply.github.com>