Delete generated test cases, move testData to boxWithStdlib

compiler/testData/codegen/boxWithStdlib/dataClasses/tostring/mixedParams.kt

@@ -0,0 +1,7 @@
+data class A(var x: Int, y: Int, val z: Int?)
+
+fun box(): String {
+    val a = A(1, 2, null)
+    if("$a" != "A(x=1, z=null)") return "$a"
+    return "OK"
+}