test: Remove insignificant_test from tst_qtiff
test: Remove insignificant_test from tst_qtiff

This test is now passing on all CI platforms.

Task-number: QTBUG-28320
Change-Id: I6e0977980cfe788a087646eb751b113f83e8dd12
Reviewed-by: Tony Sarajärvi <9f5d03eac1d5fa677086d4fb7148df11e4536984@digia.com>
Reviewed-by: aavit <7b3193bdca3a57b9140f9c882d4fe8f629e8932f@digia.com>

kobolabs/qtimageformats,kobolabs/qtimageformats,kobolabs/qtimageformats,kobolabs/qtimageformats