pt-kill: Add a --log-dsn option.

This is customer issue 23049.

t/lib/Processlist.t

@@ -9,7 +9,7 @@ BEGIN {
 use strict;
 use warnings FATAL => 'all';
 use English qw(-no_match_vars);
-use Test::More tests => 32;
+use Test::More tests => 33;
 
 use Processlist;
 use PerconaTest;
@@ -600,6 +600,17 @@ my %find_spec = (
    },
 );
 
+my $matching_query =
+      {  'Time'    => '91',
+         'Command' => 'Query',
+         'db'      => undef,
+         'Id'      => '43',
+         'Info'    => 'select * from foo',
+         'User'    => 'msandbox',
+         'State'   => 'executing',
+         'Host'    => 'localhost'
+      };
+
 my @queries = $pl->find(
    [  {  'Time'    => '488',
          'Command' => 'Connect',
@@ -675,33 +686,24 @@ my @queries = $pl->find(
          'State'   => 'Locked',
          'Host'    => 'localhost'
       },
-      {  'Time'    => '91',
-         'Command' => 'Query',
-         'db'      => undef,
-         'Id'      => '43',
-         'Info'    => 'select * from foo',
-         'User'    => 'msandbox',
-         'State'   => 'executing',
-         'Host'    => 'localhost'
-      },
+      $matching_query,
    ],
    %find_spec,
 );
 
-my $expected = [
-      {  'Time'    => '91',
-         'Command' => 'Query',
-         'db'      => undef,
-         'Id'      => '43',
-         'Info'    => 'select * from foo',
-         'User'    => 'msandbox',
-         'State'   => 'executing',
-         'Host'    => 'localhost'
-      },
-   ];
+my $expected = [ $matching_query ];
 
 is_deeply(\@queries, $expected, 'Basic find()');
 
+{
+   # Internal, fragile test!
+   is_deeply(
+      $pl->{_reasons_for_matching}->{$matching_query},
+      [ 'Exceeds busy time', 'Query matches Command spec', 'Query matches Info spec', ],
+      "_reasons_for_matching works"
+   );
+}
+
 %find_spec = (
    busy_time    => 1,
    ignore => {