Simplify backend terminate and wait logic in postgres_fdw test

When Who What
2021-04-15 05:36:44 Bharath Rupireddy (bharathrupireddy) Closed in commitfest 2021-07 with status: Committed
2021-04-12 05:34:41 Michael Paquier (michael-kun) Added michael-kun as committer
2021-04-12 05:34:39 Michael Paquier (michael-kun) Added michael-kun as reviewer
2021-04-09 14:15:43 Bharath Rupireddy (bharathrupireddy) Changed authors to Bharath Rupireddy (bharathrupireddy)
2021-04-09 14:15:35 Bharath Rupireddy (bharathrupireddy) Attached mail thread CALj2ACXGY_EfGrMTjKjHy2zi-u1u9rdeioU_fro0T6Jo8t56KQ@mail.gmail.com
2021-04-09 14:15:35 Bharath Rupireddy (bharathrupireddy) Created patch record