[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/kt42208b.kt

@@ -25,7 +25,8 @@ inline fun foo() = {
 // test.kt:6 box
 
 // EXPECTATIONS JS_IR
+// test1.kt:10 box
 // test.kt:4 box
 // test.kt:5 box
 // test1.kt:7 box$lambda
-// test.kt:6 box
+// test.kt:6 box