Update tests

compiler/testData/codegen/box/typeMapping/nothing.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JS_IR
 fun box(): String {
     // kotlin.Nothing should not be loaded here
     val x = "" is Nothing

compiler/testData/codegen/box/typeMapping/nullableNothing.kt

@@ -1,6 +1,13 @@
-// IGNORE_BACKEND: JS_IR
+
+var result = "fail"
+
+fun sideEffect(): Any {
+    result = "OK"
+    return Unit
+}
+
 fun box(): String {
     // This used to be problematic because of an attempt to load kotlin/Nothing class
-    val x = "" is Nothing?
-    return "OK"
+    val x = sideEffect() is Nothing?
+    return result
 }