tests/test-blockjob: remove exit callback

tests/test-blockjob: remove exit callback

We remove the exit callback and the completed boolean along with it.
We can simulate it just fine by waiting for the job to defer to the
main loop, and then giving it one final kick to get the main loop
portion to run.

Signed-off-by: John Snow <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 20180906130225[email protected]
Reviewed-by: Jeff Cody <[email protected]>
Signed-off-by: Max Reitz <[email protected]>