test: add in a regression test for #13691 (#17507)

Looks like there was already another test linked to this, but this adds
in the original example to fully close the issue.

[skip community_build]

close #13691

Author: nicolasstucki

tests/run/i13691b.scala

@@ -0,0 +1,12 @@
+// https://github.com/lampepfl/dotty/issues/13691
+import language.experimental.saferExceptions
+
+trait Decoder[+T]:
+  def apply(): T
+
+given Decoder[Int throws Exception] = new Decoder[Int throws Exception]:
+  def apply(): Int throws Exception = 1
+
+@main def Test(): Unit =
+  import unsafeExceptions.canThrowAny
+  summon[Decoder[Int throws Exception]]()