MDEV-15686: Loading MyRocks plugin back after it has been unloaded causes a crash

Adjust the testcase to handle all possible outcomes.

storage/rocksdb/mysql-test/rocksdb/r/mariadb_plugin.result

@@ -16,5 +16,16 @@ UNINSTALL SONAME 'ha_rocksdb';
 call mtr.add_suppression("Plugin 'ROCKSDB.*' init function returned error.");
 call mtr.add_suppression("Plugin 'ROCKSDB.*' registration as a INFORMATION SCHEMA failed.");
 call mtr.add_suppression("Plugin 'ROCKSDB' registration as a STORAGE ENGINE failed");
+#
+# There are two possible scenarios:
+#  ha_rocksdb.{dll,so} is still loaded into mysqld's address space. Its
+#  global variables are in the state that doesn't allow it to be
+#  initialized back (this is what MDEV-15686 is about). This is handled 
+#  by intentionally returning an error from rocksdb_init_func.
+#
+#  The second case is when ha_rocksdb.{ddl,so} has been fully unloaded 
+#  and so it will be now loaded as if it happens for the first time.
 INSTALL SONAME 'ha_rocksdb';
-ERROR HY000: Internal error: Loading MyRocks plugin after it has been unloaded is not supported. Please restart mysqld
+# Whatever happened on the previous step, restore things to the way they
+# were at testcase start.
+UNINSTALL SONAME 'ha_rocksdb';

storage/rocksdb/mysql-test/rocksdb/t/mariadb_plugin.test

@@ -34,5 +34,22 @@ call mtr.add_suppression("Plugin 'ROCKSDB.*' init function returned error.");
 call mtr.add_suppression("Plugin 'ROCKSDB.*' registration as a INFORMATION SCHEMA failed.");
 call mtr.add_suppression("Plugin 'ROCKSDB' registration as a STORAGE ENGINE failed");
 
---error ER_INTERNAL_ERROR
+--echo #
+--echo # There are two possible scenarios:
+
+--echo #  ha_rocksdb.{dll,so} is still loaded into mysqld's address space. Its
+--echo #  global variables are in the state that doesn't allow it to be
+--echo #  initialized back (this is what MDEV-15686 is about). This is handled 
+--echo #  by intentionally returning an error from rocksdb_init_func.
+--echo #
+--echo #  The second case is when ha_rocksdb.{ddl,so} has been fully unloaded 
+--echo #  and so it will be now loaded as if it happens for the first time.
+
+--error 0,ER_INTERNAL_ERROR
 INSTALL SONAME 'ha_rocksdb';
+
+--echo # Whatever happened on the previous step, restore things to the way they
+--echo # were at testcase start.
+--error 0,ER_SP_DOES_NOT_EXIST
+UNINSTALL SONAME 'ha_rocksdb';
+