Document use of expected-failures files in acceptance tests

[phil-davis]

WHAT Needs to be Documented?

Acceptance tests can be run with an expected-failures file. That file records tests that are known to be failing, and links to related issues. Document the test workflow of how we make use of that.

WHERE Does This Need To Be Documented (Link)?

https://doc.owncloud.com/server/next/developer_manual/testing/acceptance-tests.html

WHY Should This Change Be Made?

Help developers to know what is the current workflow for acceptance tests.

(Optional) What Type Of Content Change Is This?

• New Content Addition
• Old Content Deprecation
• Existing Content Simplification
• Bug Fix to Existing Content

(Optional) Which Manual Does This Relate To?

• Admin Manual
• Developer Manual
• User Manual
• Android
• iOS
• Branded Clients
• Desktop Client
• Other

[phil-davis]

Note: owncloud/core#39782 added a lint check of expected-failures files. So include that in the documentation. Probably also add a manual make command in the relevant repo/repos to allow developers to easily run a local lint check of the expected failures file(s).