[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/kt42208c.kt

@@ -38,6 +38,7 @@ fun baz(v:(() -> Unit)) {
 // test.kt:7 box
 
 // EXPECTATIONS JS_IR
+// test1.kt:11 box
 // test.kt:4 box
 // test3.kt:14 baz
 // test1.kt:8 box$lambda
@@ -48,4 +49,4 @@ fun baz(v:(() -> Unit)) {
 // test3.kt:14 baz
 // test1.kt:8 box$lambda
 // test3.kt:15 baz
-// test.kt:7 box
+// test.kt:7 box