MDEV-27791: rocksdb_log_dir test postfix

We can only remove a subdirectory in mtr on an installed instance

Example failure previously:

CURRENT_TEST: rocksdb.rocksdb_log_dir
mysqltest: At line 15: Path '/usr/local/mariadb-10.9.0-linux-systemd-x86_64/mysql-test/var/tmp/1' is not a subdirectory of MYSQLTEST_VARDIR '/usr/local/mariadb-10.9.0-linux-systemd-x86_64/mysql-test/var/1'or MYSQL_TMP_DIR '/usr/local/mariadb-10.9.0-linux-systemd-x86_64/mysql-test/var/tmp/1'

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

@@ -12,4 +12,4 @@ DROP DATABASE mysqltest;
 --list_files $MYSQL_TMP_DIR *LOG
 
 # Clean up
-remove_files_wildcard $MYSQL_TMP_DIR *LOG;
+remove_files_wildcard $MYSQL_TMP_DIR/./ *LOG;