qt,tests: Make test pass on 32-bit systems

* lang/qt/tests/t-addexistingsubkey.cpp
(AddExistingSubkeyJobTest::testAddExistingSubkeyWithExpiration): Handle
negative expiration date.

On 32-bit systems the expiration date of the test key overflows. This
will cause the AddExistingSubkeyJob to fail. We expect it to fail with
an "invalid time" error.

• GnuPG-bug-id: T6137