Insufficient test coverage of op+(string-ish,string-ish)

The QStringBuilder test comes close, but it's really unsystematic and hard to see which combinations are being tested and which ones aren't. We should rewrite `runScrenario()` into a template function (but keep `P` and `Q`), and drive it from per-combination Q_SLOTS a la tst_qstringapisymmetry.cpp.