add biginteger tests + mod change

[ixje]

What current issue(s) does this address, or what feature is it adding? Add new tests for a block that failed. Change implementation to use Decimal % thanks to @merl

How did you solve this problem? see above How did you make sure your solution works?

Are there any special changes in the code that we should be aware of?

Please check the following, if applicable:

• [ ] Did you add any tests?
• [X] Did you run make lint?
• [X] Did you run make test?
• [X] Are you making a PR to a feature branch or development rather than master?
• [ ] Did you add an entry to CHANGELOG.rst? (if not, please do)

[coveralls]

Coverage increased (+0.004%) to 85.28% when pulling 0b935cd7dbffb935f86c860678d0aa7169bcc6bd on ixje:bigint4 into 3817375e17566b2364ff8c459d2407618569abd6 on CityOfZion:development.