Mark test traits as @internal

TaxonomyTestTrait has two methods that are directly related to generating entities. They are inconsistent from the perspective of "I want to create an entity, potentially with some values".

function createVocabulary() {

function createTerm(Vocabulary $vocabulary, $values = []) {

The question came up on ✨ Allow raw vid to be sorted in views Fixed about whether I was breaking BC by adding a $values to createVocabulary(). At the moment it is true that someone in a project could extend TaxonomyTestTrait, they could override createVocabulary() with createVocabulary($some_string); and thus if I added $values to the core method I might break their tests.

First, I want to suggest that ENTITYTestTraits should be marked @internal (see below). But if we want to improve re-usability of such traits, and avoid BC issues, we should define some standards right now. So i"m just throwing this out there to see what sticks...

1. All core entities should have a consistent `ENTITYTestTrait` trait.
2. Each test trait should have a consistent `createENTITY()` method (and perhaps other CRUD?)
3. A general $values array should always be the FIRST argument to `createENTITY($value)` method.
4. A test should ensure these interfaces are consistent (or perhaps the existence of tests using these interfaces is enough)

Wait, what? They're just test helpers, and test classes/methods are not in scope for BC... (I'm assuming, but hey we don't actually *test* them.)

So maybe these traits should be marked as @internal? Since they are explicitly only for testing - only for re-usability of code in testing - should they be excluded from the API? Let's say I subclassed a core test and started using it's methods, does that mean the core tests need to maintain their own internal method interfaces?