[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/smapInlineAsInlineArgument.kt

@@ -38,6 +38,7 @@ fun fail() : String {
 // test.kt:9 box
 
 // EXPECTATIONS JS_IR
+// test.kt:16 box
 // test.kt:4 box
 // test.kt:5 box
 // test.kt:20 fail
@@ -47,4 +48,4 @@ fun fail() : String {
 // test.kt:16 box
 // test.kt:8 box
 // test.kt:16 box
-// test.kt:9 box
+// test.kt:9 box