test: add in regression test for #14830 (#17514)

[skip community_build]

closes #14830

Author: nicolasstucki

tests/pos/i14830.scala

@@ -0,0 +1,6 @@
+
+// https://github.com/lampepfl/dotty/issues/14830
+val a: Comparable[String] = "Fred"
+val b: { def length: Int } = "Fred"
+val c: Comparable[String] & { def length: Int } = "Fred"
+val d: Comparable[String] & { def length(): Int } = "Fred"