Put libpq_pipeline cancel test back

Put libpq_pipeline cancel test back

I disabled it in cc6e64afda53 because it was unstable; hopefully the
changes here make it more stable.  (Jelte proposed to submit the queries
using simple query protocol, but I chose to instead make the monitor
connection wait until the PgSleep wait event is seen.  This is probably
not a terribly meaningful increase in coverage ...)

Discussion: https://postgr.es/m/CAGECzQRQh_5tSy+5cndgv08eNJ2O0Zpwn2YddJtSsmC=Wpy1BQ@mail.gmail.com