Wait until the task is done in test_task::finish_failed and
test_task::finish to ensure that it is folded into its parent.
(cherry picked from commit 30f97ea133)
Have a private about_source for every module
and request abort on stop() to signal all outstanding
tasks to abort (especially when they are sleeping
for the task_ttl).
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>