Prove that rendering to a cubemap both face-by-face and with MRT are working as expected with QRhi

Add a manual test, similarly to QTBUG-81213.

Must verify that we can

• render to a cubemap's 6 faces one by one, in 6 separate draw calls
• render to a cubemap's all 6 faces in one go (on systems where the number of max color attachments is > 4)