Add Github Actions CI (bp #75)

[mergify]

This is an automated backport of pull request #75 done by Mergify

Cherry-pick of 4b5f5ae has failed:

On branch mergify/bp/1.2.x/pr-75
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	deleted by us:   .github/workflows/test.yml

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[jackkoenig]

@chick 1.2.x didn't have CI so Mergify/git was kind enough to turn this backport into just adding CI 🙂

[jackkoenig]

@chick a few tests fail with errors like:

[info] HierarchicalModulesExample:
[info] - This is an example of a module with hierarchical submodules  *** FAILED ***
[info]   java.io.IOException: Cannot run program "xdg-open": error=2, No such file or directory

Why isn't this an issue on 1.3.x?

[chick]

@jackkoenig It's a complete mystery, I'm trying to track this down now. xdg-open is mentioned only in one unused function in FirrtlDiagrammer object. I'm looking for clues still

[chick]

I thought this would be fixed by PR #80 which forced the diagram open program to "" for the tests.
But CI still fails