Removed underscores from test method names to be consistent with other components

This is just a cosmetic change. It is more common (in Symfony) to use fully camel-cased names for test methods. Only some of the test methods contain underscores. To avoid confusion it is better to be consistent.

Bug fix: no
Feature addition: no
Backwards compatibility break: no
Fixes the following tickets: -
Todo: -
License of the code: MIT
Documentation PR: -