Add tests for produce-block-offset-ms

[linh2931]

This comes from review discussion from #1800 for future work.

This is not new behavior, it has shipped a block off early since v2.0.4. See EOSIO/eos#8651
We should consider refactoring producer_plugin so it is easier to test. Or it might be possible to use mock_time like the test_trx_retry_db does. However, I believe that is out of scope for this PR. Please create a GitHub issue to work that later.