Add tests for deferred completion

[ocadaruma]

Motivation

• In https://github.com/line/decaton/pull/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    • Decaton doc's example code was also outdated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 (https://github.com/line/decaton/pull/124)
  • => We should add integration test cases to ensure it works
• Besides, I fixed ProcessorTestSuite to await retried-produced tasks to be committed