Enable strict config schema for tests

Problem/Motivation

Currently, we have this code setup in "MenuMultilingualContextTest" and "MenuMultilingualContextMenuBlockTest":
~~~
/**
* {@inheritdoc}
*/
protected $strictConfigSchema = FALSE;
~~~
This is against the Drupal Coding Standards and should be removed (setting it to TRUE):

Do not disable strict config schema checking in tests. Instead, ensure your module properly declares its schema for configurations.

Steps to reproduce

Proposed resolution

Remove the code and verify the tests once again.

Remaining tasks

User interface changes

API changes

Data model changes