[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/smapInlineAsInfixArgument.kt

@@ -36,6 +36,7 @@ fun fail() : String {
 // test.kt:11 box
 
 // EXPECTATIONS JS_IR
+// test.kt:14 box
 // test.kt:6 box
 // test.kt:18 fail
 // test.kt:6 box
@@ -45,4 +46,4 @@ fun fail() : String {
 // test.kt:14 box
 // test.kt:9 box
 // test.kt:3 execute
-// test.kt:11 box
+// test.kt:11 box