GitHub Actions runs tests twice

[clone1018]

Since we have both a mix test and mix coveralls our tests end up running twice, we could save half the time by figuring out how to consolidate it.

[dmarcoux]

Dropping mix test should be enough, no?

I see from the GitHub workflow configure that it apparently fails with a 500 error from time to time. This could be addressed by splitting the generation of the code coverage data and the upload of that data. The first step would be to run mix coveralls.json, then the Coveralls GitHub action would upload the data generated by that mix task.