Use `check_added_monitors` test utility in invoice utils tests

Use `check_added_monitors` test utility in invoice utils tests

In a coming commit, the `lightning-invoice::utils` module will move
to the `lightning` crate, causing its tests to be included in the
global lockorder tests done in that crate. This should be fine,
except that the `lightning-invoice::utils` module currently holds
the `added_monitors` lock too long causing lockorder violations.

Instead, this commit replaces the legacy monitors-added test with
the `check_added_monitors` test utility.