bugfix: Fix tests using newer given syntax

Author: tgodzik

tests/neg/i19414.scala

@@ -9,7 +9,7 @@ class Printer
 given Writer[JsValue] = ???
 given Writer[JsObject] = ???
 
-given [B: Writer] => (printer: Printer = new Printer) => BodySerializer[B] = ???
+given [B: Writer](using printer: Printer = new Printer): BodySerializer[B] = ???
 
 def f: Unit =
   summon[BodySerializer[JsObject]] // error: Ambiguous given instances