Extend the shfmt-to-shasta functionality

[gkapetanakis]

This PR does a few (closely related) things:

• Fixes a bug that existed with the shfmt-to-shasta translation (which I believe might have been caused by shfmt updating to a new version? not sure)
• Improves that module mainly by making the following two changes:
  • Bash test expressions ([[ ... ]]) are now properly translated to CondNodes instead of CommandNodes
  • "|&" pipelines are now translated to PipeNodes with the proper redirections attached to them
• Adds unit tests for (I believe) all possible translations (which will fail if the aforementioned bug ever manifests again)
• Adds the unit test to the GitHub 'test' workflow

[angelhof]

This looks reasonable but could you check why the CI/CD tests fail first?