JS: move expressions test to box tests

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

@@ -0,0 +1,12 @@
+package foo
+
+fun box(): String {
+    val a = "abc"
+    val b = "abc"
+    val c = "def"
+
+    if (a != b) return "fail1"
+    if (a == c) return "fail2"
+
+    return "OK"
+}