[JS IR] Improve stepping test executor

Don't search for box function in the generated code;
instead, stop the execution right before calling it.

This makes stepping tests more accurate.

compiler/testData/debug/stepping/smapInlineInIntrinsicArgument.kt

@@ -37,10 +37,11 @@ fun fail() : String {
 // test.kt:9 box
 
 // EXPECTATIONS JS_IR
+// test.kt:12 box
 // test.kt:4 box
 // test.kt:16 fail
 // test.kt:7 box
 // test.kt:16 fail
 // test.kt:12 box
 // test.kt:7 box
-// test.kt:9 box
+// test.kt:9 box