How can I disable xb_doublewrite?
You have to look in my.cnf for the variable  innodb_parallel_doublewrite_path
You should also set this
[mysqld]
innodb_doublewrite=0

Then, restart mysql
After restarting mysql, you can then delete /test/mysql/xb_doublewrite
CAVEAT
What is the purpose of this feature ?
According to Percona Server 5.7 performance improvements

Parallel doublewrite buffer. For ages, MySQL has had only one
doublewrite buffer for flushing data pages. So even if you had several
threads for flushing you couldn’t efficiently use them – doublewrite
quickly became a bottleneck. We’ve changed that by attaching two
doublewrite buffers to each buffer pool instance: one for each type of
page flushing (LRU and flush list). This completely avoids any
doublewrite contention, regardless of the flusher thread count. We’ve
also moved the doublewrite buffer out of the system tablespace so you
can now configure its location.

So, if you do want it gone, remove innodb_parallel_doublewrite_path if it is there. Although not recommnded, you must add innodb_doublewrite=0 to disable the InnoDB Double Write Buffer altogether.