test: load Mroonga plugin by default

Without this, the following error is occurred when one MySQL server runs
two tests:

    errno: 0 service 'thd_alloc_service' interface version mismatch:

The error isn't occurred when one MySQL server runs only one test.

It means that plugin load/unload is a problem. But why?

test/run-sql-test.sh

@@ -220,6 +220,8 @@ mysql_test_run_args="${mysql_test_run_args} --parallel=${n_processors}"
 mysql_test_run_args="${mysql_test_run_args} --retry=1"
 mysql_test_run_args="${mysql_test_run_args} --suite=${test_suite_names}"
 mysql_test_run_args="${mysql_test_run_args} --force"
+mysql_test_run_args="${mysql_test_run_args} --mysqld=--loose-plugin-load-add=ha_mroonga.so"
+mysql_test_run_args="${mysql_test_run_args} --mysqld=--loose-plugin-mroonga=ON"
 if [ -n "$test_names" ]; then
     mysql_test_run_args="${mysql_test_run_args} --do-test=${test_names}"
 fi