Fix compiler warnings
KSelectionProxyModelTest is currently failing on kf6 and I was hoping that fixing the compiler warnings would do something about that. It's still failing, but here are some changes that make kitemmodels compile cleanly. Commit messages below.
Benchmark tests: Use #if defined(...) instead of #ifdef ...
The advantage is that the #elif/#else branches can be flattened into the same single #if branch per test. No more weird nesting.
Fix unused parameter ("count") warning in kdescendantsproxymodeltest
Implementing functionality for "count", even if there's only a single call site with count == 1, isn't any more code than adding a Q_UNUSED() and a comment.
Fix compiler warnings in KSelectionProxyModel and BiHashBenchmarks
The kitemmodels module compiles clean after this on my system.