MDEV-38385 Galera test failure on MDEV-30418

[plampio]

Fix for the sporadically failing MTR test galera.MDEV-30418.

The test set incorrect values (on purpose) for wsrep_cluster_address system variable.
Later in the test a correct value was restored, but there was a small time interval
after the restore during which WSREP would not allow any SELECT statements.
This caused the MTR test to fail sporadically.

Now the test waits with a SHOW statement for the WSREP to become ready for
for all SQL statements before proceeding.

[janlindstrom]

Looks good

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}