test: add in regression test for #7445

Author: ckipp01

tests/pos/i7445.scala

@@ -0,0 +1,9 @@
+// https://github.com/lampepfl/dotty/issues/7445
+object Main {
+  type O1[A] = {
+    type OutInner[X] = Unit
+    type Out = OutInner[A]
+  }
+
+  def f1: O1[Int]#Out = ???
+}