I suggest another approach.  Create a separate test database and apply
event trigger on it.  wait_for_catchup() and others use 'postgres'
database and wouldn't touch our test database.
I also added check for successful clearance of the flag on both
primary and standby.  One issue spotted there: in-place heap update
doesn't issue a WAL flush.  But I think that's minor, WAL could be
flushed by any subsequent operation.