test: add in a regression test for #13216 (#17503)

[skip community_build]

closes #13216

Author: nicolasstucki

tests/run/i13216.scala

@@ -0,0 +1,11 @@
+// https://github.com/lampepfl/dotty/issues/13216
+import scala.annotation.targetName
+
+class C(s: String) extends AnyVal {
+  def m(xs: Seq[Int]): Unit = {}
+  @targetName("m_seq2")
+  def m(xs: Seq[Seq[Int]]): Unit = {}
+}
+
+@main def Test =
+  new C("").m(Seq(123))