Use latest_throttled in cask livecheck audit

I recently modified `bump-cask-pr` to include basic throttle handling
(in line with the approach for formulae) but forgot to update
`audit_livecheck_version` to also handle the `latest_throttled` value
from `latest_version`. As it stands, `brew audit` will fail with an
error like "Version '1.2.5' differs from '1.2.6' retrieved by
livecheck" when `bump-cask-pr` tries to update to a throttled version
but a newer unthrottled version exists upstream, preventing the
autobump workflow from updating casks that use `throttle`.

This addresses the issue by updating `audit_livecheck_version` to also
handle the `:latest_throttled` value from `latest_version`, instead of
checking the unthrottled `:latest` value. Casks that aren't throttled
will continue to check the `:latest` value, as expected.

In the process, this expands tests for `audit_livecheck_version` to
increase coverage to 100%.