Specify a new client_plugindir path, to be used during testing

When testing installed packages, the client (mariadb or mysql) can have
plugins stored in a separate location compared to server. This change
ensures that the correct paths are used for the plugin-dir directive in
the my.cnf generated by mysql-test-run.

The directives in my.cnf will be replaced like so:

[client]
plugin-dir=$client_plugindir

[mysqld]
plugin-dir=$plugindir

where $<variable-name> corresponds to the variable name in mtr.

mysql-test/lib/My/ConfigFactory.pm

@@ -216,6 +216,7 @@ my @mysqld_rules=
 my @client_rules=
 (
  { 'character-sets-dir' => \&fix_charset_dir },
+ { 'plugin-dir' => sub { $::client_plugindir } },
 );