[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/conjunction.kt

@@ -34,4 +34,11 @@ fun getD() = true
 
 // EXPECTATIONS JS_IR
 // test.kt:3 box
-// test.kt:11 box
+// test.kt:13 getA
+// test.kt:4 box
+// test.kt:15 getB
+// test.kt:5 box
+// test.kt:17 getC
+// test.kt:9 box
+// test.kt:3 box
+// test.kt:11 box