Export QMailThreadPrivate, as it uses QPrivatelyImplemented.

Functionally similar to a84920f307cd4c5661f09dc41e562ff949d6779c. Not sure why
it was missed, but I can't build on Apple clang without this.

Change-Id: Id7b4b593bc761169333cca9c619b2ab77cb3561c
Reviewed-by: John Brooks <john.brooks@dereferenced.net>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Inherit tst_QCop from QCopAdaptor, as it uses some protected API.

Change-Id: I94538e1ea298977aef6e7b949c1d29e8686a9896
Reviewed-by: John Brooks <john.brooks@dereferenced.net>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>