test: add in regression test for #5700

Author: ckipp01

tests/pos/i5700.scala

@@ -0,0 +1,5 @@
+// https://github.com/lampepfl/dotty/issues/5700
+object noRecursionLimit:
+  type M = { type T[+A]; type Ev >: T[Any] <: T[Nothing] }
+  val M: M = ().asInstanceOf[M]
+  def dcast(m: M.T[Any]): M.T[Int] = m: M.Ev