[2.7] unsilence deprecations but prevent them from breaking the tests

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12973
| License       | MIT
| Doc PR        | -

This will show deprecations in the logs of the tests, but still allow them to be green.
This is not done on each component on purpose.
This is the first step to make 2.7 free from any deprecation notice.
Next steps are:

1. remove all deprecation warnings that are related to 2.7 *wrongly* using deprecated interfaces. That will still leave notices related to BC layers.
2. isolate all tests related to BC layers in their own "Legacy"-prefixed test classes.
3. switch `off` then `on` again deprecations using `error_reporting` in setUp/tearDown for each of these legacy- test classes
4. revert this PR and put back error_reporting to E_ALL in every phpunit.xml.dist file

This process will:
- make tests green again,
- ensure over time that "pure" 2.7 API does not rely on deprecated one
- ensure over time that the 2.7 BC-layer keeps working
Shouldn't this change be merged into older versions as well?