ktest.pl: Add the log of last test in email on failure

If a failure happens and an email is sent, show the contents of the log of
the last test that failed in the email.

Link: http://lore.kernel.org/r/20200701231756.619246244@goodmis.org



Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>