Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/regressions/kt4262.kt

@@ -0,0 +1,12 @@
+// WITH_RUNTIME
+
+fun <E : Enum<E>> Byte.toEnum(clazz : Class<E>) : E =
+    (clazz.getMethod("values").invoke(null) as Array<E>)[this.toInt()]
+
+enum class Letters { A, B, C }
+
+fun box(): String {
+    val clazz = Letters::class.java
+    val r = 1.toByte().toEnum(clazz)
+    return if (r == Letters.B) "OK" else "Fail: $r"
+}