Fix declarative_core unit test

There are still some bugs caused by V4 but more dedicated bugs but it is
more important to get the current unit test running . We rather skip the
problematic cases.

A lot of warnings have been fixed as well.

Task-number: QTBUG-31798
Change-Id: I1fccf26db5c47563baeaf6c2d6366d3c10aee824
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>
Stop skipping declarative_core tests.

These test cases were failing due to regressions in QML and have since
been fixed. See QTBUG-33542, QTBUG-33546 and QTBUG-33561.

Change-Id: Id86e9a8baa29eea851bda27cb668a6ce586c5f25
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>