Speed up mariabackup.xb_compressed_encrypted

With system mutexes, contention can be very expensive.
Let us configure innodb_encryption_threads=1 to minimize contention.
The actual work is being done in buf_flush_page_cleaner thread anyway.

mysql-test/suite/mariabackup/xb_compressed_encrypted.opt

 --innodb-encryption-rotate-key-age=2
---innodb-encryption-threads=4
+--innodb-encryption-threads=1
 --innodb-tablespaces-encryption
 --plugin-load-add=$FILE_KEY_MANAGEMENT_SO
 --loose-file-key-management