Add tests for deferred completion

[ocadaruma]

Motivation

• In Support deferred completion timeout as a workaround for "leak" #99, we made some spec change about ProcessingContext#deferCompletion() that:
  • It requires to re-use Completion instance returned from deferCompletion() if we want to refer it later, otherwise completion will be leaked
  • However we realized that this change could cause a lot of migration cost for Decaton <= 2.x users
    • Decaton doc's example code was also outdated
  • So we concluded to make keep existing deferCompletion-usage to work by adding fix (Leave completion object in ProcessingContext field to avoid leak #124)
  • => We should add integration test cases to ensure it works
• Besides, I fixed ProcessorTestSuite to await retried-produced tasks to be committed

[kawamuray]

I'm +1 for adding test case to check this usage is valid, but IMO we should not advertise this as the primary usage of deferred completion, because unless users require ProcessingContext later in async processing, they should prefer using only Completion so that memory pressure is dramatically reduced?

[ocadaruma]

they should prefer using only Completion so that memory pressure is dramatically reduced

Good point. I'll revert this and add another test cases with caution.

[kawamuray]

LGTM, thanks 👍