JS: move expressions test to box tests

js/js.translator/testData/box/expression/when/whenExecutesOnlyOnce.kt

@@ -0,0 +1,16 @@
+package foo
+
+class A() {
+
+}
+
+fun box(): String {
+    var a = 0
+    when(A()) {
+        is A -> a++;
+        is A -> a++;
+        else -> a++;
+    }
+    if (a != 1) return "fail: $a"
+    return "OK"
+}