[JS IR] Adapt stepping tests for Kotlin/JS

compiler/testData/debug/stepping/primitiveNullChecks.kt

@@ -6,7 +6,14 @@ fun box(): String {
     return "OK"!!
 }
 
-// EXPECTATIONS
+// EXPECTATIONS JVM JVM_IR
 // test.kt:4 box
 // test.kt:5 box
-// test.kt:6 box
+// test.kt:6 box
+
+// EXPECTATIONS JS_IR
+// test.kt:4 box
+// test.kt:5 box
+// test.kt:5 box
+// test.kt:6 box
+// test.kt:6 box