Fix the linking of tst_qwebenginepage on Mac

Skip Mac specific tst_QWebEnginePage::macCopyUnicodeToClipboard()
which uses the unimplemented QWebEnginePage::setHtml.

Change-Id: I39c2d0d7b61517646e1702ca4f1ac119daef1585
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>