JS backend: added jsCode test cases

js/js.translator/testData/jsCode/cases/for.kt

@@ -0,0 +1,18 @@
+package foo
+
+fun factorial(n: Int): Int = js("""
+    var result = 1;
+
+    for (var i = 1; i <= n; i++) {
+        result *= i;
+    }
+
+    return result;
+""")
+
+fun box(): String {
+    assertEquals(24, factorial(4))
+    assertEquals(120, factorial(5))
+
+    return "OK"
+}