DX: Test that Transformers are adding only CustomTokens that they define and nothing else

[keradus]

The transformer classes are internal, so we can do whatever we want there without BC promise and upcoming BC breaks warnings.

Let's use the getCustomTokens method to ensure that Transformer is adding only allowed custom tokens, and no other transformer is adding them.

Unfortunately, to implement this check i had to bypass few things (eg access internals), but I couldn't find a good way to do it without refactoring main Tokenizer class into first-class service.

[coveralls]

Coverage remained the same at 91.929% when pulling 635068c on keradus:2.18_CUSTOM_TOKENS into e2c6a1a on FriendsOfPHP:2.18.