test: fix flaky debugger test

Author: islandryu

test/parallel/test-debugger-break.js

@@ -53,6 +53,7 @@ const cli = startCLI(['--port=0', script]);
     'marks the debugger line');
 
   await cli.command('sb("break.js", 6)');
+  await cli.waitFor(/> 6.*[.\s\S]*debug>/);
   assert.doesNotMatch(cli.output, /Could not resolve breakpoint/);
 
   await cli.command('sb("otherFunction()")');