gallery-app tests should wait until toolbar autohides before continuing

[Jean-Baptiste Lallement]

The current test passes even if the toolbar cannot be revealed and reports false positives. For example on mako build #94 [1] 2 test_camera_button_visible passes and 1 failed while they should all have failed.

For a non-automated user, the application in its current state is not usable and the testsuite should be almost all red, instead of giving a false sense of confidence.

The testsuite should be fixed so that all the tests using the toolbar wait until the toolbar autohides, then reveal it.

[1] http://ci.ubuntu.com/smokeng/utopic/touch/mako/94:20140623:20140530/8685/gallery_app/