Fix some badly written Galera tests

galera.galera_kill_applier: Make the test less likely to fail
by adding sleep time.

galera.query_cache: Remove data truncation.

Part of the test file looks like it has been misinterpreted as latin1
and wrongly converted to UTF-8 encoding. In MariaDB 10.1, the server would
only warn about data truncation and not issue an error. 10.2 is stricter.
(The test should be carefully reviewed if it really makes sense.)