ConfigValidation class contains code that is brittle and changing for every addition

Problem/Motivation

Every time we add new things to validate and test it using config_test.validate we're changing \Drupal\config_test\ConfigValidation::validateMapping(). See #2920682: Add config validation for plugin IDs and #2920678: Add config validation for the allowed characters of machine names

This is a test only change.

Proposed resolution

Make the validation less brittle to change.

This was added in #1928868-102: Typed config incorrectly implements Typed Data interfaces - the comment that lead to test was:

So we currently do not validate an entire mapping or sequence on its own.

So this change is still ensuring that we are looking at all the keys in the mapping.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None