- Added suppressions for dlopen to make plugin test pass.
- Do not pass empty string to mysqld, since my_getopt is
  not capable to handle it.
- Re-enabled trailing UNINSTALL PLUGIN statement of plugin.test.

The memory leak described in the bug report happens in libdl, not
in mysqld. On some valgrind installations this error is suppressed
by default, no idea why it isn't suppressed on pb-valgrind.

<observation>
If library remains open after thread has finished, and is closed by
another thread, we get memory leak. But in case library is opened and
closed by the same thread no leak occurs.
</observation>


mysql-test/lib/mtr_io.pl:
  Do not pass empty string since my_getopt is not capable to handle it.
  This is required to make plugin.test work on platforms where
  ha_example is not compiled.
mysql-test/r/plugin.result:
  Updated test result.
mysql-test/t/plugin.test:
  Re-enabled UNINSTALL PLUGIN statement. It is a good idea to cleanup
  at the end of the test. Anyway bug22694 was fixed while this test
  was disabled.
mysql-test/valgrind.supp:
  Added suppressions for dlopen to make plugin test pass.