Added pre and post option to test run (allows me to adjust an engine once all of the data has been loaded).

Kinda obvious that this was going to come up ;)


client/client_priv.h:
  New options
client/mysqlslap.c:
  Added option for pre/post run
mysql-test/r/mysqlslap.result:
  Updated tests
mysql-test/t/mysqlslap.test:
  Updated test