[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/for.kt

@@ -31,4 +31,5 @@ inline fun foo(n: Int) {}
 // test.kt:3 box
 // test.kt:3 box
 // test.kt:3 box
-// test.kt:6 box
+// test.kt:3 box
+// test.kt:6 box