JS: move more test to box tests

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

@@ -0,0 +1,18 @@
+package foo
+
+fun testIf(flag: Boolean): Int = js("""
+    if (flag)
+        return 1;
+    else
+        return -1;
+
+    return 0;
+""")
+
+
+fun box(): String {
+    assertEquals(1, testIf(true))
+    assertEquals(-1, testIf(false))
+
+    return "OK"
+}