qt,tests: Make sure expiration time is interpreted as unsigned number

* lang/qt/tests/t-addexistingsubkey.cpp,
lang/qt/tests/t-changeexpiryjob.cpp: Convert expiration time to
uint_least32_t.

This doesn't change the outcome of the tests (they also pass without
this change because of the expiration dates of the test keys), but it's
still good practise to treat the expiration time as an unsigned number
if the assertions check that the expiration time is in some range.

• GnuPG-bug-id: T6137