fix cluster test instability

[sanikolaev]

There's a workaround in https://github.com/manticoresoftware/manticoresearch-php/blob/master/test/Manticoresearch/ClusterTest.php#L103 :

        // workaround against unstable tests. For some reason the replication which
        // has to be synchronous acts like if it was asynchronous
        sleep(3);

without it it has a high chance to fail which looks wrong since given Manticore has synchronous replication it should be stable without the sleep(N). The task is to figure out what's going on and create another issue in https://github.com/manticoresoftware/manticoresearch/ if the problem is on that end.

[sanikolaev]

The sleep(3) approach may be not the best one, but okay. Manticore's replication is actually virtually synchronous (as Mariadb and Galera call it), so in case of slow runners (which github actions runners may be) the instability may be ok and some pause may be required to increase the stability.