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.
a1cac6d54b