Let mtr lookup the plugins, rather than hard-coding autotools lib path.


mysql-test/include/not_parallel.inc:
  Add new feature for skipping when running in parallel.
mysql-test/mysql-test-run.pl:
  Use find_plugin to locate plugins.
  Export opt_parallel in environment.
mysql-test/suite/federated/federated_plugin-master.opt:
  Use plugin dir from mtr.
mysql-test/suite/federated/federated_plugin.test:
  Skip test, disabled.
mysql-test/t/archive_plugin-master.opt:
  Use plugin dir from mtr.
mysql-test/t/archive_plugin.test:
  Skip test if running in parallel.
mysql-test/t/blackhole_plugin-master.opt:
  Use plugin dir from mtr.
mysql-test/t/blackhole_plugin.test:
  Skip test if running in parallel.