[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/simpleInlineDefaultArg.kt

@@ -38,3 +38,6 @@ fun box(): String {
 // test.kt:10 box
 
 // EXPECTATIONS JS_IR
+// test.kt:3 box
+// test.kt:5 box
+// test.kt:6 box