JS: move more test to box tests

js/js.translator/testData/box/number/numberIncDec.kt

@@ -0,0 +1,16 @@
+package foo
+
+fun box(): String {
+
+    assertEquals(100.inc(), 101)
+    assertEquals(100.dec(), 99)
+
+    var x = 100
+    assertEquals(x.inc(), 101)
+    assertEquals(x, 100)
+
+    assertEquals(x.dec(), 99)
+    assertEquals(x, 100)
+
+    return "OK"
+}