Avoid use of NOTICE to wait for snapshot invalidation

Avoid use of NOTICE to wait for snapshot invalidation

This idea (implemented in commits and bc32a12e0db2 and 9e8fa05d3412) of
using notices to detect that a session is sleeping was unreliable, so
simplify the concurrency controller session to just look at
pg_stat_activity for a process sleeping on the injection point we want
it to hit.  This change allows us to remove a secondary injection point
and the alternative expected output files.

Reproduced by Alexander Lakhin following a report in buildfarm member
skink (which runs the server under valgrind).

Author: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/3e302c96-cdd2-45ec-af84-03dbcdccde4a@gmail.com