psql: Make cancel test more timing robust

psql: Make cancel test more timing robust

The previous coding relied on the PID file appearing and the query
starting "fast enough", which can fail on slow machines.  Also, there
might have been an undocumented interference between alarm and
IPC::Run.  This new coding doesn't rely on any of these concurrency
mechanisms.  Instead, we wait unitl the PID file is complete before
proceeding, and then also wait until the sleep query is registered by
the server.

Discussion: https://www.postgresql.org/message-id/flat/E1mH14Q-0002gh-HS%40gemulon.postgresql.org