Plan A:

I don't have a solution to your Question, but contemplate the following alternative.

1. Toss the test nodes.
2. Take one of the production nodes out of the production cluster.
3. Turn that node into the first test node.
4. Then, as you are doing currently, populate the other 4 test nodes.

Meanwhile, use SST to rebuild the stolen production node.  (I don't know whether to do this first, last, or in parallel.)

Plan B:

Another thought...  Use LVM.  But this probably means having separate disk partitions (or physical drives) for production and test.  Or maybe a single partition, but with 2 live snapshots!

If you could snapshot _all_ the nodes _simultaneously_, you could get an identical copy of production into test in a minute or two.  Unfortunately, this would require a complex, coordinated, very brief, outage.

Back to a hybrid.

1. Take one node out of the production cluster.
2. Perform LVM snapshot (a minute or so)
3. Put that node back into production.  (IST will quickly resync.)
4. Now use the snapshot as the first node of Test.
5. SST to get rest of Test.