Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/increment/memberExtOnLong.kt

@@ -0,0 +1,14 @@
+// WITH_RUNTIME
+
+object ExtProvider {
+    operator fun Long.get(i: Int) = this
+    operator fun Long.set(i: Int, newValue: Long) {}
+}
+
+fun box(): String {
+    with (ExtProvider) {
+        var x = 0L
+        val y = x[0]++
+        return if (y == 0L) "OK" else "Failed, y=$y"
+    }
+}