test_camera_multiple_shots - Workaround for marionette is_displayed

Some tweaks to get this working on the Camera app again as Marionette is not letting us use `is_displayed` on the filmstrip anymore

@teodosia can you check this on v1-train for me?

`master` -> Tested OK, merge
`v1-train` -> Affected but only merge the test fix commit not the unxfail commit
`v1.0.1` -> Do not merge