Temporarily disable tst_QJSEngine tests to allow qtbase dev integration

Temporarily disables:
- tst_QJSEngine::dateConversionJSQt()
- tst_QJSEngine::dateConversionQtJS()

so that https://codereview.qt-project.org/#change,65560 can merge.

Change-Id: I415daf1c5bd137e1905773f88fe5ebaaab1e0a06
Reviewed-by: John Layt <jlayt@kde.org>