MDEV-23375 parts.partition_debug fails when it's run after another test

Make sure system tables aren't open, as the test kills the server

mysql-test/suite/parts/r/partition_debug.result

+FLUSH TABLES;
 #
 # Bug#13737949: CRASH IN HA_PARTITION::INDEX_INIT
 # Bug#18694052: SERVER CRASH IN HA_PARTITION::INIT_RECORD_PRIORITY_QUEUE

mysql-test/suite/parts/t/partition_debug.test

@@ -8,6 +8,10 @@
 # Crash tests don't work with embedded
 --source include/not_embedded.inc
 
+# Make sure system tables are not open, as the test will kill the server
+# and it will cause corruption errors in the log
+FLUSH TABLES;
+
 # Partitioning test that require debug features
 
 --echo #