add regression test for pattern match on path-dependent type

this was fixed recently (by #16827), but the test case is simpler
than the one in the PR. minimized from user code on the Typelevel
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Author: SethTisue

tests/pos/t16827.scala

@@ -0,0 +1,9 @@
+// scalac: -Werror
+
+trait Outer[F[_]]:
+  sealed trait Inner
+  trait Inner1 extends Inner
+  def foo(rv: Either[Inner, Int]) =
+    rv match
+      case Right(_) =>
+      case Left(_) =>