Add test showing use of @nowarn

Author: odersky

tests/pos/given-loop-prevention.scala

@@ -0,0 +1,14 @@
+//> using options -Xfatal-warnings
+
+class Foo
+
+object Bar {
+  given Foo with {}
+  given List[Foo] = List(summon[Foo]) // ok
+}
+
+object Baz {
+  @annotation.nowarn
+  given List[Foo] = List(summon[Foo]) // gives a warning, which is suppressed
+  given Foo with {}
+}