Fix bloom WAL tap test

Hi!

I just realized that these lines of contrib/bloom/t/001_wal.pl don't check
that queries give same results on master and standby. They just check that
*return codes* of psql are equal.

# Run test queries and compare their result
> my $master_result = $node_master->psql("postgres", $queries);
> my $standby_result = $node_standby->psql("postgres", $queries);
> is($master_result, $standby_result, "$test_name: query result matches");

Attached patch fixes this problem by using safe_psql() which returns psql
output instead of return code. For safety, this patch replaces psql() with
safe_psql() in other places too.

I think this should be backpatched to 9.6 as bugfix.

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Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company