Refactor background psql TAP functions

Refactor background psql TAP functions

This breaks out the background and interactive psql functionality into a
new class, PostgreSQL::Test::BackgroundPsql.  Sessions are still initiated
via PostgreSQL::Test::Cluster, but once started they can be manipulated by
the new helper functions which intend to make querying easier.  A sample
session for a command which can be expected to finish at a later time can
be seen below.

  my $session = $node->background_psql('postgres');
  $bsession->query_until(qr/start/, q(
    \echo start
	CREATE INDEX CONCURRENTLY idx ON t(a);
  ));
  $bsession->quit;

Patch by Andres Freund with some additional hacking by me.

Author: Andres Freund <[email protected]>
Reviewed-by: Andrew Dunstan <[email protected]>
Discussion: https://postgr.es/m/20230130194350[email protected]