JS: move more test to box tests

js/js.translator/testData/box/jsCode/codeFromVariable.kt

@@ -0,0 +1,14 @@
+package foo
+
+fun box(): String {
+    var c: Int = 0
+
+    val code = "c = 3"
+    js(code)
+
+    assertEquals(3, c)
+    js(("c = 5"))
+
+    assertEquals(5, c)
+    return "OK"
+}