toaster: add getMessage to MockEvent
The MockEvent needs to not only stand in for Toaster and Bitbake quick events,
it also needs to stand in for LogRecord, and for that it needs to provide
the new getMessage method.
[YOCTO #11440]
Signed-off-by: David Reyna <David.Reyna@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>