Test: fix occasional 069.memory_leak_extended test failure.

Test: fix occasional 069.memory_leak_extended test failure.

It turned out that reason of the occasional test failure is, pgpool
child process is gone before running ps command after pgbench
finishes. The cause is a kind mismatch FATAL error, "DISCARD ALL
cannot be executed within a pipeline". To fix this, run pgbench in
background and get the process size before pgbench finishes.

Discussion: https://www.pgpool.net/pipermail/pgpool-hackers/2023-May/004338.html