Update and activate tst_animators

Amends a0f8be40

; it's not clear if
there's any reason we haven't been running this test all these years.
But we need to use testFileUrl() for loading QML files, and some
additional updates make it more like the other tests.

Change-Id: I3ddcc993c4536c2d4d751a37c0425943c7d86766
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
(cherry picked from commit ec9d125f

)
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>