Improve Code Coverage

[kittybest]

Prerequisites

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

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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 #932)
(previous closed issue is this: #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/