Add tests for QtConcurrent::mappedReduced() with different types for result and intermediate

Documentation says it's possible to define the reduce function as follows:

V function(T &result, const U &intermediate)

where T is the type of the final result, U is the return type of the map function.

However, all our tests operate only on cases where T is the same as U.

Please check the qtbase/tests/auto/concurrent/qtconcurrentmap/functions.h. The last class there "NumberSumReduce" is prepared for the test, however, there are no tests for it inside tst_qtconcurrentmap.cpp.

I've just checked that it works OK, so adding tests for it would be useful.