test: fix comparing bool to int in cluster.test.py

In commit c1c77782
("replication: fix bootstrap failing with ER_READONLY")
seek_once was changed to seek_wait. Seek_once returned a non-negative int
on success and -1 if failed, seek_wait returns True on success and
False if failed. Therefore no need to compare it to 0 anymore.

NO_DOC=Minor fix in test
NO_CHANGELOG=Minor fix in test

(cherry picked from commit ed01ac1a)