Made tst_QQmlDebugJS insignificant on Mac.

Made tst_QQmlDebugJS insignificant on Mac.

This test has been failing often on Mac. It seems that test case only
fails with Qt5 master integration, but not with QtDeclarative CI. This
test has now failed twice consecutive in virtualized Mac OSX machines.
Since I'm going to add some more virtual Macs to CI tomorrow morning,
making this test case insignificant to avoid good changes integrating
in additional capacity.

For more information, see the following bug report.

Task-number: QTBUG-28263

Change-Id: I3492373f4f034c27fdbc207e395a40ef96dccbd9
Reviewed-by: Samuli Piippo <samuli.piippo@digia.com>
Reviewed-by: Simo Fält <simo.falt@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>