Rename *.jet -> *.kt in codegen tests

compiler/testData/codegen/classes/rightHandOverride.kt

@@ -0,0 +1,20 @@
+// Changed when traits were introduced. May not make sense any more
+
+trait Left {}
+open class Right() {
+    open fun f() = 42
+}
+
+class D() : Left, Right() {
+    override fun f() = 239
+}
+
+fun box() : String {
+    val r : Right = Right()
+    val d : D = D()
+
+    if (r.f() != 42) return "Fail #1"
+    if (d.f() != 239) return "Fail #2"
+
+    return "OK"
+}