Currently, the three-node mtr suite for Galera (galera_3nodes)
uses a separate IPv6 availability check using the "have_ipv6.inc"
file. This check duplicates a more accurate check at suite.pm
level, which can be used by including the file "check_ipv6.inc".

This patch removes this discrepancy between suites. In addition,
one of the files in the galera_3nodes suite does not contain the
option "--bind-address=::" which is needed for the test to work
correctly with IPv6 (at least on some systems), since without
it the server will not wait for connections on the IPv6 interface.

https://jira.mariadb.org/browse/MDEV-18379