tst_QAtomicInteger: fix UBs (signed overflow)

Drop addSub() test. It executes exactly the union of fetchAndAdd()
and fetchAndSub(), which have already had their UBs fixed.

No need to do fixes in duplicated code.

Change-Id: Ib72caab0310fce3ff9a40c261d8a38518f91ecaf
Reviewed-by: David Faure <david.faure@kdab.com>