htlcswitch: use mock clock in TestMailBoxAddExpiry

This commit replaces the clock used in the unit test
`TestMailBoxAddExpiry`. Previously the `TestClock` is used, resulting in
the unit test being not so "unit" as the maintainer needs to know the
detailed implementation of `clock.Clock`, resulting in debugging a
failed unit test more difficult as the cognitive cost is high.
Re-implement `clock.Clock` also means we need to maintain more. This is
now solved by using mock clock so we can ignore the implementation
details and care only the returned results.

docs/release-notes/release-notes-0.16.0.md

@@ -205,6 +205,9 @@ certain large transactions](https://github.com/lightningnetwork/lnd/pull/7100).
 * [Code style cleanup](https://github.com/lightningnetwork/lnd/pull/7308) in the
   funding package.
 
+* [Fixed a flake in the TestMailBoxAddExpiry unit
+  test](https://github.com/lightningnetwork/lnd/pull/7314).
+
 ## `lncli`
 
 * [Add an `insecure` flag to skip tls auth as well as a `metadata` string slice