[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/ifWithInlineInCondition.kt

@@ -56,6 +56,7 @@ fun nop() {}
 // test.kt:21 box
 
 // EXPECTATIONS JS_IR
+// test.kt:24 box
 // test.kt:3 box
 // test.kt:4 box
 // test.kt:30 nop
@@ -74,4 +75,4 @@ fun nop() {}
 // test.kt:15 box
 // test.kt:19 box
 // test.kt:30 nop
-// test.kt:21 box
+// test.kt:21 box