`void_checks` should have tests with assignment patterns

[srawlins]

Child issue of #58837

I think there should be a test case like:

void f(void p) {
  (p, ) = (7, );
}

in which the lint rule would report that assigning to p violates void_checks.

Also maybe something with object patterns? like class C<T> { T f; } and case C<void>(f: var x): print('hi');...

[pq]

Nice catch!