JS: move expressions test to box tests

js/js.translator/testData/box/expression/equals/equalsNullOrUndefined.kt

@@ -0,0 +1,23 @@
+package foo
+
+fun box(): String {
+    val a: Int? = null
+    val r = a == null
+    if (!r || a != null)
+        return "wrong result on simple nullable check"
+
+    //force using Kotlin.equals
+    val t = null
+    if (t != undefined)
+        return "wrong result when compare null and undefined using Kotlin.equals"
+
+    var i = 0;
+    fun foo(): Int? = ++i;
+    if (foo() == null)
+        return "wrong result on nullable check with side effects"
+
+    if (i != 1)
+        return "wrong affects when using nullable check with side effects"
+
+    return "OK"
+}