This commit explicitly does **not** include adding a parameter type to the `File::getTokensAsString()` `$start` and `$length` parameters as the pre-existing inline parameter validation is more comprehensive and more reliable.

Includes changing the expected parameter type for a couple of methods from `DomNode` to `DomElement`.
The previous was incorrect, but changing the method signature by making a type more specific, is a breaking change, so had to wait until now.

This commit explicitly does **not** include adding a parameter type to the `MessageCollector::add()` `$message` and `$type` parameters and the `Common::getSniffCode()` `$sniffsClass` parameter, as the pre-existing inline parameter validation is more comprehensive and more reliable.

Includes removing a type check from an "internal-only" class which is no longer necessary (and updating the associated test).

Includes updating the sniff test classes overloading these methods.