Fix -source for neg test

Author: odersky

Diff for: tests/neg/given-triangle.check

@@ -1,4 +1,4 @@
--- [E172] Type Error: tests/neg/given-triangle.scala:14:18 -------------------------------------------------------------
-14 |@main def Test = f  // error
+-- [E172] Type Error: tests/neg/given-triangle.scala:15:18 -------------------------------------------------------------
+15 |@main def Test = f  // error
    |                  ^
    |Ambiguous given instances: both given instance given_B and given instance given_C match type A of parameter a of method f

Diff for: tests/neg/given-triangle.scala

@@ -1,3 +1,4 @@
+//> using -source 3.5
 class A
 class B extends A
 class C extends A