Change integration to acceptance tests

[phil-davis]

Addresses issue #3854 resulting from core PR owncloud/core#30493

1. Modify the folder name to reflect the actual new name.
2. Refer to these tests as "acceptance" tests. This distinguishes them from other places in the documentation that talk about the type of integration test that uses multiple classes together - example https://github.com/owncloud/documentation/blob/master/developer_manual/app/tutorial/testing.rst
3. Update the folder structure and a few other wordings that I noticed.

Later, as we move toward putting the UI tests in this acceptance folder, we will need to sort out more different words to use to describe the different categories of acceptance tests that we have - API end-to-end, UI end-to-end etc.

[settermjd]

LGTM!

[phil-davis]

This is dev/tester doc. It is implemented in core master and stable10, and in master of any repos that have these kind of tests. It is not something that is "released" in the usual sense, so it is "live" now.

Therefore this [can|should] be merged.