Use the coveralls Github action

[bmerry]

Also use it with parallel flags, which will hopefully stop the MacOS coverage builds from failing.

[coveralls]

coverage: 94.354% (+1.6%) from 92.721%
when pulling 5089f17 on coveralls-action
into 59b8e55 on master.

[amishatishpatel]

Apologies for the delay, I had to read up on GitHub Actions syntax/keywords. Looks fantastic, as always. I have to ask before explicit Approval (genuinely, because I haven't dabbled in the Actions before)

• How have you confirmed this change works?

[bmerry]

Apologies for the delay, I had to read up on GitHub Actions syntax/keywords. Looks fantastic, as always. I have to ask before explicit Approval (genuinely, because I haven't dabbled in the Actions before)

* How have you confirmed this change works?

Yep: https://coveralls.io/builds/70463282