kunit: example: Provide example exit functions

Add an example .exit and .suite_exit function to the KUnit example
suite. Given exit functions are a bit more subtle than init functions
(due to running in a different kthread, and running even after tests or
test init functions fail), providing an easy place to experiment with
them is useful.

Reviewed-by: Rae Moar <rmoar@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>