The current set of TAP tests for two-phase transactions include some
coverage for post-commit callbacks of multixact, but it lacked tests for
testing the recovery of those callbacks.  This commit adds some tests
with soft and hard restarts in this case, using transactions which
include DDLs.

Author: Michael Paquier
Reviewed-by: Oleksii Kliukin
Discussion: https://postgr.es/m/20190221055431.GO15532@paquier.xyz