test: re-enable linearizable test after fix

The test flaked a lot on FreeBSD and Mac OS due to some proxy problems
which were fixed in scope of tarantool/luatest@5b704ac.

Another possible failure looked like this:

NO_WRAP
[006] not ok 5  linearizable-read.test_leader_change
[006] #   Can't modify data on a read-only instance - it is an orphan
[006] #   stack traceback:
[006] #         ...tarantool/test/replication-luatest/linearizable_test.lua:196: in function 'linearizable-read.test_leader_change'
[006] #         ...
[006] #         [C]: in function 'xpcall'
NO_WRAP

Fix it by waiting for the server to follow its upstreams.

Closes tarantool/tarantool-qa#277

NO_DOC=test fix
NO_CHANGELOG=test fix