Testing: Stub all oEmbed requests as empty string

[aduth]

Related: #10682

This pull request seeks to improve end-to-end tests to stub an empty HTML response for all oEmbeds.

Testing Instructions:

Ensure end-to-end tests pass:

npm run test-e2e

[danielbachhuber]

I've created #10710 as a follow-up task.

[notnownikki]

I think the demo test needs to be left as it is. The reason is that we check if the demo content marks the post as dirty, so we need to get back a response from Vimeo, removing the source from the iframe but preserving the dimensions, so that we can check that aspect ratio detection isn't going to make the demo page dirty when it loads. Other tests can stub out the response to an empty string, but the demo page needs to test that we're not going to make everyone's demo post dirty.

[danielbachhuber]

I think the demo test needs to be left as it is. The reason is that we check if the demo content marks the post as dirty, so we need to get back a response from Vimeo, removing the source from the iframe but preserving the dimensions, so that we can check that aspect ratio detection isn't going to make the demo page dirty when it loads.

@notnownikki How do you suggest we handle the intermittent failure that's failing everyone's builds right now and blocking development?

[notnownikki]

@danielbachhuber could we have the part which stubs out the iframe content respond with a mock if the actual vimeo embed request fails?

[notnownikki]

Changing my review on this after discussing at https://wordpress.slack.com/archives/C02QB2JS7/p1539819998000100

I'll fix up the demo test to use a mock if vimeo is down, and the actual response if it isn't, in a future PR.

[aduth]

I don't know that this solution is viable. Closing with #11116 as parent issue for tracking.