[debugger] add test case for stepping into string template.

test/mjsunit/debug-stepin-builtin-callback-opt.js

Index: test/mjsunit/debug-stepin-builtin-callback-opt.js
diff --git a/test/mjsunit/debug-stepin-builtin-callback-opt.js b/test/mjsunit/debug-stepin-builtin-callback-opt.js
index 6b0039674c2bb12a21fa6e1599bb87ea96be4e4f..737bfc894631dfdf3eeede1e22ef60a231a90255 100644
--- a/test/mjsunit/debug-stepin-builtin-callback-opt.js
+++ b/test/mjsunit/debug-stepin-builtin-callback-opt.js
@@ -13,7 +13,8 @@ function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     print(event_data.sourceLineText());
-    assertTrue(event_data.sourceLineText().indexOf(`Break ${break_count++}.`) > 0);
+    assertTrue(
+        event_data.sourceLineText().indexOf(`Break ${break_count++}.`) > 0);
     exec_state.prepareStep(Debug.StepAction.StepIn, 1);
   } catch (e) {
     exception = e;