Suggested patch is in attached diff.txt

wsrep_sst_mariabackup is suggested to be just wrapper around wsrep_sst_xtrabackup-v2 , (which requires few changes to wsrep_sst_xtrabackup-v2)

function innobackupex() {

  mariabackup --innobackupex "$@"

}

function xbstream() {

  mbstream "$@"

}

. "$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"/wsrep_sst_xtrabackup-v2

It has new test sst_smoke , which does simple sst attempt (only with rsync and mariabackup at the moment, needs mysqldump to be added after proper fix in MDEV-13968 and MDEV-13969). (Because current sst tests are huge and require --big-test and work only in debug build)
It has some patches addressed to hide problems in MDEV-13968 and MDEV-13969, so will conflict with commits from them - but needs them to make sure that new test does work properly.

Actually proposed here sst_smoke test will not be needed if patch from MDEV-14035 is approved, which introduces smoke sst together with more complex combinations for each sst
EDIT: It is still better to add proposed sst_smoke, because sst_mysqldump in MDEV-14035 uses some complicated configuration with redefining receive_address, etc and starts with rsync, then changes to mysqldump method dynamically, etc.
And this test doesn't have any non-default option (no .cnf file)

as far as tests are concerned, both galera_sst_xtrabackup-v2 and galera_sst_mariabackup tests were fixed to pass, there's no need to merge them.