Simplify TAP test structure to align with upstream (#169)

postgres/postgres@549ec20 moved to using "done_testing()" from
the Perl testing framework, which removed the need to include test
counts.

test/t/004_centers.pl

@@ -2,7 +2,7 @@ use strict;
 use warnings;
 use PostgresNode;
 use TestLib;
-use Test::More tests => 3;
+use Test::More;
 
 # Initialize node
 my $node = get_new_node('node');
@@ -34,3 +34,5 @@ $node->safe_psql("postgres",
 
 # Test no error for duplicate centers
 test_centers(10);
+
+done_testing();