[tests-only] [full-ci] Refactor previews tests so they can run on oC10 or OCIS

[phil-davis]

Description

There are various previews.feature tests that are not being skipped on OCIS. And in the OCIS repo there are versions of some of these tests that are "bug demos". These days we have the expected-failures files. So we can run the "real" tests from core in OCIS CI and add them to expected-failures if they do not pass.

This PR:

1. Removes the skip tags from all the previews tests, so that they will all run in all CI
2. Provides two different test scenarios for sizing of previews of files downloaded through the webdav API - the behavior of oC10 is that the exact requested size is returned (even if the request is something "weird" like 1 pixel by 32 pixels). OCIS behaves differently (and more reasonably, depending on how you define "reasonable" - it always returns a width-height that preserves the aspect-ratio of the original file). That is discussed in different output preview dimension from oc10 backend ocis#2069 - and we are accepting that the 2 implementations can have this difference in behavior.

Related Issue

owncloud/ocis#2313

How Has This Been Tested?

CI

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[SwikritiT]

LGTM

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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