JS: move expressions test to box tests

js/js.translator/testData/box/expression/misc/safeCallComputesExpressionOnlyOnce.kt

@@ -0,0 +1,14 @@
+package foo
+
+var i = 0
+
+fun test(): Int? = i++
+
+fun box(): String {
+    if (i != 0) return "fail1: $i"
+    test()?.plus(1)
+    if (i != 1) return "fail2: $i"
+    test()?.minus(2)
+    if (i != 2) return "fail3: $i"
+    return "OK"
+}