Improve Code Coverage

[kittybest]

Prerequisites

Please answer the following question for yourself before submitting a feature request.

• [x] I checked to make sure that this request has not already been filed

Motivation

There are still conditions not being tested, and we only know them until someone runs under certain condition and report the bug. Wanna find a way to reduce this problem.

Details

Currently we test the code coverage by nyc (which was done in https://github.com/privacy-scaling-explorations/maci/pull/932) (previous closed issue is this: https://github.com/privacy-scaling-explorations/maci/issues/931)

Try to improve testing the code coverage more automatically and completely by codecov

Make sure that you have reviewed MACI's Contributor Guidelines

[kittybest]

problems to be discussed: codecov charges monthly subscription fee, do we really need it? or we only need nyc?

[samajammin]

problems to be discussed: codecov charges monthly subscription fee, do we really need it? or we only need nyc?

Looks like free for open source projects, so we should be good? https://about.codecov.io/pricing/