Having two different TestWebChannel classes in qtwebchannel is confusing

Acceptance criteria:

• Rename the actual QtTest test class to `tst_QWebChannel` or something similar.
• Update the friend declaration to the idiomatic way so the test class doesn't get a different name in -qt-namesspace builds:
  • forward-declare the tst_ class outside QT_BEGIN_NAMEPACE
  • use friend class ::tst_X; (the :: is important)