test: rename cluster to replica_set in gh_6260 test

luatest_helpers/cluster module was recently added to luatest trunk and
renamed to replica_set.
Let's update its name everywhere in gh_6260_add_builtin_events_test,
since this test will be amended in the following commits and the new
module name will be used.

In-scope-of #5272

NO_DOC=refactoring
NO_CHANGELOG=refactoring