no_leading_underscores_for_local_identifiers tests should include variable patterns

[srawlins]

Child issue of #58837

I think there should be a test case like:

switch ((1, 2)) {
  case (var _a, var _b): ...
}

in which the lint rule would report that the variables shouldn't start with underscore.

Also a destructuring example:

(int?, int?) position = ...

var (_x, _y) = position;

[pq]

Thanks a million Sam!