TestLib: Fix eager optional dereference in initIntegrationTestCase

QVERIFY2 evaluates its message argument eagerly, so
.arg(*intended, ...) dereferenced a disengaged optional
whenever initIntegrationTestMain() was not called (i.e. every
QTEST_MAIN integration test). Debug builds abort there;
release builds read garbage into a discarded string.

Resolve the expected backend name with value_or() first.

Pick-to: 6.12
Change-Id: Ia7592f387259d47d6ba2102cb5aa83b4e68b6422
Reviewed-by: Nils Petter Skålerud <nils.petter.skalerud@qt.io>