Use 'Drupal.NamingConventions.ValidFunctionName' in core/tests

Kinda same as the other one I reviewed, leaning on the tests are green and the phpcs rule catching anything missed. The changes themselves make sense though.

Are we sure about this change? There is a PHP class \DOMXPath. I'm not sure that this is a good rule to implement in core for two reasons.
1. PHP does not care about case for method names. If a class has a method getXPath() and I call getxpath() it'll work just fine.
2. We might be forcing developers to make bad choices for no reason. For example, say you work for the BBC and you implementing a method getBBCListing() - there is probably a corporate standard that says BBC must always be capitalised but our coding standards prevent that. I'm not saying I particularly like the method name getBBCListing() but I'm not sure that our coding standards should have an opinion. Also if you are extending PHP's classes you have things like \DOMNode::C14N and \DOMNode::C14NFile.

Postponed the other 2 so main conversation can happen here.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

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