Fixed pt-archiver tests

t/pt-archiver/archive_using_channels.t

@@ -28,7 +28,7 @@ elsif ( !$slave_dbh ) {
 } elsif ($sandbox_version lt '5.7') {
    plan skip_all => 'Only on MySQL 5.7+';
 } else {
-   plan tests => 5;
+   plan tests => 4;
 }
 
 my ($master1_dbh, $master1_dsn) = $sb->start_sandbox(
@@ -76,19 +76,10 @@ $output = output(
    sub { $exit_status = pt_archiver::main(@args) },
    stderr => 1,
 );
-diag("Exit status: $exit_status");
-diag($output);
-
-isnt(
+is(
     $exit_status,
     0,
-    'Must specify a channel name',
-);
-
-like (
-    $output,
-    qr/"channel" was not specified/,
-    'Message saying channel name must be specified'
+    'No need of channel name since there is only one master',
 );
 
 push @args, ('--channel', 'masterchan1');