on MySQL 5.5:

'CURRENT_TIMESTAMP' is not supported as a default value for 'datetime' type

on MySQL 5.7:
'CURRENT_TIMESTAMP' is supported for 'datetime' type, but not for 'datetime(6)' - unfortunately, Hibernate uses datetime(6) for java.util.Date

We would probably have to switch to timestamps instead of dates/calendars, but that could break the test in the other databases.
I would suggest to skip the test on MySQL 5 dialects rather than add ugly hacks to make it work..