Delete generated test cases, move testData to boxWithStdlib

compiler/testData/codegen/boxWithStdlib/dataClasses/multiDeclaration.kt

@@ -0,0 +1,7 @@
+data class A(val x: Int, val y: Any?, val z: String)
+
+fun box(): String {
+    val a = A(42, null, "OK")
+    val (x, y, z) = a
+    return if (x == 42 && y == null) z else "Fail"
+}