Updated mtr help

- Updated combination example to use `innodb_plugin`
```
$ ./mysql-test/mtr rpl.rpl_invoked_features # no rpl.rpl_invoked_features,mix,xtradb_plugin

worker[1] Using MTR_BUILD_THREAD 300, with reserved ports 16000..16019
rpl.rpl_invoked_features 'innodb_plugin,mix' [ pass ]    904
rpl.rpl_invoked_features 'mix,xtradb'    [ pass ]   1707
rpl.rpl_invoked_features 'innodb_plugin,row' [ pass ]    927
rpl.rpl_invoked_features 'row,xtradb'    [ pass ]    828
rpl.rpl_invoked_features 'innodb_plugin,stmt' [ pass ]    855
rpl.rpl_invoked_features 'stmt,xtradb'   [ pass ]    952
--------------------------------------------------------------------------
```
- `gdb` option can take optional gdb arguments (good documentation in
KB)

mysql-test/mysql-test-run.pl

@@ -6088,7 +6088,7 @@ Examples:
 
 alias
 main.alias              'main' is the name of the suite for the 't' directory.
-rpl.rpl_invoked_features,mix,xtradb_plugin
+rpl.rpl_invoked_features,mix,innodb_plugin
 suite/rpl/t/rpl.rpl_invoked_features
 
 Options to control what engine/variation to run:
@@ -6216,7 +6216,7 @@ Options for debugging the product
   debug-server          Use debug version of server, but without turning on
                         tracing
   debugger=NAME         Start mysqld in the selected debugger
-  gdb                   Start the mysqld(s) in gdb
+  gdb[=gdb_arguments]   Start the mysqld(s) in gdb
   manual-debug          Let user manually start mysqld in debugger, before
                         running test(s)
   manual-gdb            Let user manually start mysqld in gdb, before running