Improve contrib/amcheck's tests for CREATE INDEX CONCURRENTLY.

Commits fdd965d07 and 3cd9c3b92 tested CREATE INDEX CONCURRENTLY by
launching two separate pgbench runs concurrently.  This was needed so
that only a single client thread would run CREATE INDEX CONCURRENTLY,
avoiding deadlock between two CICs.  However, there's a better way,
which is to use an advisory lock to prevent concurrent CICs.  That's
better in part because the test code is shorter and more readable, but
mostly because it automatically scales things to launch an appropriate
number of CICs relative to the number of INSERT transactions.
As committed, typically half to three-quarters of the CIC transactions
were pointless because the INSERT transactions had already stopped.

In passing, remove background_pgbench, which was added to support
these tests and isn't needed anymore.  We can always put it back
if we find a use for it later.

Back-patch to v12; older pgbench versions lack the
conditional-execution features needed for this method.

Tom Lane and Andrey Borodin

Discussion: https://postgr.es/m/139687.1635277318@sss.pgh.pa.us

src/test/perl/PostgreSQL/Test/Cluster.pm

@@ -2070,54 +2070,6 @@ sub pgbench
 
 =pod
 
-=item $node->background_pgbench($opts, $files, \$stdout, $timer) => harness
-
-Invoke B<pgbench> and return an IPC::Run harness object.  The process's stdin
-is empty, and its stdout and stderr go to the $stdout scalar reference.  This
-allows the caller to act on other parts of the system while B<pgbench> is
-running.  Errors from B<pgbench> are the caller's problem.
-
-The specified timer object is attached to the harness, as well.  It's caller's
-responsibility to select the timeout length, and to restart the timer after
-each command if the timeout is per-command.
-
-Be sure to "finish" the harness when done with it.
-
-=over
-
-=item $opts
-
-Options as a string to be split on spaces.
-
-=item $files
-
-Reference to filename/contents dictionary.
-
-=back
-
-=cut
-
-sub background_pgbench
-{
-	my ($self, $opts, $files, $stdout, $timer) = @_;
-
-	my @cmd =
-	  ('pgbench', split(/\s+/, $opts), $self->_pgbench_make_files($files));
-
-	local %ENV = $self->_get_env();
-
-	my $stdin = "";
-	# IPC::Run would otherwise append to existing contents:
-	$$stdout = "" if ref($stdout);
-
-	my $harness = IPC::Run::start \@cmd, '<', \$stdin, '>', $stdout, '2>&1',
-	  $timer;
-
-	return $harness;
-}
-
-=pod
-
 =item $node->connect_ok($connstr, $test_name, %params)
 
 Attempt a connection with a custom connection string.  This is expected