Add test coverage report for Pull Requests

[gtxu]

For now we have a unit test for all component before PR get merged, but we don't have unit test coverage report for a single PR. There are tools (such as Coveralls)that automatically post unit test coverage report for a PR in comment. This may be a considerable option for stargz-snapshotter.

[gtxu]

An example of coveralls:

[vkuzniet]

Hi @ktock, what do you think about adding this to the pipeline?