tst_qqmlapplicationengine: clear $QT_MESSAGE_PATTERN

The test fails if it's set, because it looks at the output from testapp
and compares it with expected output.

Change-Id: Idc82e2f9024b72cb8be919c2d1bfa4efb4957e4e
Reviewed-by: Kai Koehne <kai.koehne@digia.com>