Fixes #1043. Add first parts of func test suite for image uploads.

[miketaylr]

This tests showing and removing image previews of screenshots that come from add-ons.

[miketaylr]

(holding off on review, I want to see if this timeout failure happens again on Travis -- this passes locally for me)

[miketaylr]

I need to look at this again after the fix for #1049 lands -- things have changed.

[miketaylr]

r? @karlcow -- we obviously need to add tests for issue image uploads and "regular" file uploads as well. probably a good task for @deepthivenkat :)

[karlcow]

grmph

First pass

× firefox 46.0.1 on MAC - Image Uploads (non-auth) - postMessaged dataURI image upload worked
× firefox 46.0.1 on MAC - Issues - Issue page loads
× firefox 46.0.1 on MAC - Search (auth) - Search from the homepage
× firefox 46.0.1 on MAC - Search (auth) - Search/filter interaction

Second pass

Let's try another one. :)

× firefox 46.0.1 on MAC - Search (auth) - Search works
× firefox 46.0.1 on MAC - Issues - Issue page loads
× firefox 46.0.1 on MAC - Image Uploads (non-auth) - postMessaged dataURI image upload worked

@miketaylr × firefox 46.0.1 on MAC - Image Uploads (non-auth) - postMessaged dataURI image upload worked seems to fail consistently. What's happening for your when running locally.

[miketaylr]

They pass for me locally. :/

Don't worry about spending more time on this for now, I'll come back to it.

[miketaylr]

(also thanks for attempting review!)

[miketaylr]

All checks have passed

(Also, they passed on Travis.)

[deepthivenkat]

Test case : Image Uploads (non-auth) - postMessaged dataURI image

1. First Test : Failed on 27th may
   https://gist.github.com/deepthivenkat/d3c7f36db8259a92327b5688ed5e6d12
2. Second Test : Passed on 29th may
   https://gist.github.com/deepthivenkat/37ffc6a09504002ba3cb7f6e749c80f6

[karlcow]

ah @deepthivenkat has a similar failure. Hypothesis. The setup we are using for working is different. Or the way we proceed for starting the tests.

Or… another silly theory… network issues, aka @miketaylr is "local" and we are "distant".

[karlcow]

I will merge because it's working on Travis, but it would be good to figure out what's happening.