MDEV-7476: Allow SELECT to succeed even when node is not ready

Added a SESSION-only system variable "wsrep_dirty_reads" to allow SELECT
queries to pass even when the node is not prepared to accept queries
(wsrep_ready=OFF). Added a test case.

mysql-test/suite/galera/suite.pm

@@ -34,6 +34,7 @@ push @::global_suppressions,
      qr(WSREP: Failed to send state UUID:),
      qr(WSREP: last inactive check more than .* skipping check),
      qr(WSREP: SQL statement was ineffective),
+     qr(WSREP: Releasing seqno [0-9]* before [0-9]* was assigned.),
    );