Status as of 31.12.2018:

1. wsrep suite test run result:

"Completed: Failed 1/11 tests, 90.91% were successful.

Failing test(s): wsrep.mdev_10186".

2. During wsrep suite running this "wsrep.variables" was skipped:

wsrep.variables                          [ skipped ]  Test requires Galera library version >= 25.3.24

But at the same run it seems that I have the version 26.4:

wsrep loader: [INFO] wsrep_load(): Galera 26.4.0(r4454) by Codership Oy <info@codership.com> loaded successfully.

Status as of 02.01.2019:

"Completed: Failed 1/11 tests, 90.91% were successful.
Failing test(s): wsrep.mdev_10186"

"wsrep.variables" is skipped probably due to this:

--let $galera_version=25.3.24

source include/check_galera_version.inc;

Status as of 03.01.2019 is the same except wsrep.variables which is not skipped now, but disabled:

"wsrep.variables [ disabled ] Global wsrep_on manipulation causes debug asserts"

Status as of 04.04.2019.
Server debug build from 10.4 branch, commit ba7d33a898d383f19f18733e6c25b029cdc304f3.

Failing test(s): wsrep.mdev_10186

Status as of 15.04.2019.
Server debug build from 10.4 branch, commit e4c5551964f398ebbe2b1b34fef028eff6f22fbe.
Tests run on SSD.

Only one failure:
wsrep.mdev_10186

Remaining open issues reported separately.