Test cleanup related to MDEV-12501

 * Removing unnecessary --plugin-maturity=unknown definitions from tests

mysql-test/include/have_plugin_auth.opt

---plugin-load-add=$AUTH_TEST_PLUGIN_SO --plugin-maturity=unknown
+--plugin-load-add=$AUTH_TEST_PLUGIN_SO

mysql-test/suite/encryption/include/have_example_key_management_plugin.opt

 --plugin-load-add=$EXAMPLE_KEY_MANAGEMENT_SO
 --loose-example-key-management
---plugin-maturity=unknown

mysql-test/suite/galera/galera_2nodes.cnf

@@ -4,7 +4,6 @@
 [mysqld]
 wsrep-on=1
 binlog-format=row
-plugin-maturity=unknown
 innodb-autoinc-lock-mode=2
 default-storage-engine=innodb
 wsrep-provider=@ENV.WSREP_PROVIDER

mysql-test/suite/galera/suite.opt

---plugin-maturity=unknown

mysql-test/suite/galera_3nodes/galera_3nodes.cnf

@@ -3,7 +3,6 @@
 
 [mysqld]
 binlog-format=row
-plugin-maturity=unknown
 innodb-autoinc-lock-mode=2
 default-storage-engine=innodb
 

mysql-test/suite/innodb/suite.opt

---plugin-maturity=unknown

mysql-test/suite/rpl/suite.opt

---plugin-maturity=unknown

storage/rocksdb/mysql-test/rocksdb/suite.opt

 --plugin-load=$HA_ROCKSDB_SO --default-storage-engine=rocksdb
---plugin-maturity=unknown

storage/rocksdb/mysql-test/rocksdb_hotbackup/suite.opt

---plugin-maturity=unknown

storage/rocksdb/mysql-test/rocksdb_rpl/suite.opt

---ignore-db-dirs=.rocksdb --plugin-load=$HA_ROCKSDB_SO --default-storage-engine=rocksdb --plugin-maturity=unknown
+--ignore-db-dirs=.rocksdb --plugin-load=$HA_ROCKSDB_SO --default-storage-engine=rocksdb

storage/rocksdb/mysql-test/rocksdb_stress/suite.opt

---plugin-maturity=unknown

storage/rocksdb/mysql-test/rocksdb_sys_vars/suite.opt

 --ignore-db-dirs=#rocksdb --plugin-load=$HA_ROCKSDB_SO
---plugin-maturity=unknown

storage/rocksdb/mysql-test/storage_engine/suite.opt

 --ignore-db-dirs=#rocksdb --plugin-load=$HA_ROCKSDB_SO --binlog_format=ROW --collation-server=latin1_bin --loose-rocksdb_flush_log_at_trx_commit=0
---plugin-maturity=unknown