JS backend: added jsCode test cases

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

@@ -0,0 +1,19 @@
+package foo
+
+fun factorial(n: Int): Int = js("""
+    var result = 1;
+    var i = 1;
+
+    while(i <= n) {
+        result *= i++;
+    }
+
+    return result;
+""")
+
+fun box(): String {
+    assertEquals(24, factorial(4))
+    assertEquals(120, factorial(5))
+
+    return "OK"
+}