Fixed flaky tests

[hpoettker]

The tests previously tested assertions against the replica after executing changes in the source without waiting for the replication to converge.

At least on my machine, the tests currently fail at least a couple of times when executed in loops like

for i in $(seq 10); do prove t/pt-table-sync/pt-1256.t; done

or

for i in $(seq 10); do prove t/pt-table-sync/char_chunking.t; done

The code is my own creation and it can be distributed under the GPL2 licence.

• The contributed code is licensed under GPL v2.0
• Contributor Licence Agreement (CLA) is signed
• util/update-modules has been ran
• Documentation updated
• Test suite update