Please verify that the --pid_suffix option works as intended

[gaogaotiantian]

Discussed in #345

^{Originally posted by kllp June 13, 2023}
Recent versions of viztracer don't seem to append the PID to the filename. Instead it creates result.json.

viztracer --pid_suffix single_process.py appends the PID in version 0.12.3 but not in 0.15.6

https://viztracer.readthedocs.io/en/latest/concurrency.html#combine-reports

[TTianshun]

Seemed that ofile with pid_suffix is not set correctly in VizUI. I can make a pr to fix it.