Add Ubuntu Build+Test to Appveyor CI

[bergmeister]

PR Summary

This is based on PR 939 (that enables build+test using PowerShell Core) and contains only small tweaks to also build and run on Ubuntu using the private beta of Appveyor.
The main difference is that ps tasks also get run as pwsh PowerShell Core tasks on Linux in AppVeyor but PowerShell Core tasks do not persist the session, therefore additional Import-Module calls were needed.
To fix the build, one rule was in a 'rules' folder instead of 'Rules', therefore the casing of the folder had to be corrected.

Note that AppVeyor for Linux ix currently in private Beta but I think the risk of breaking changes is low and will be probably minor such as e.g. a rename of the image name. Currently the Ubuntu image has the version 16.04.4 LTS

PR Checklist

Note: Tick the boxes below that apply to this pull request by putting an x between the square brackets. Please mark anything not applicable to this PR NA.

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• NA User facing documentation needed
• Change is not breaking
• NA Make sure you've added a new test if existing tests do not effectively test the code changed
• This PR is ready to merge and is not work in progress
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready

[JamesWTruher]

Generally this looks great - how do we report on the build? Does it get rolled into the current badge? Both pass or it's a fail?

[bergmeister]

In total we have 4 builds (WMF 4, WMF5, PS6 on Windows, PS6 on Ubuntu) and if any of them fail, the badge should be red. One can still click on the result and see which of the 4 builds failed and dill into it:

The only thought that I had is that although the naming of the PowerShellEdition in the yaml (the variable name itself does not matter so it is just about readability) could match what PowerShell defines as Desktop or Core. I did not know that at first but on the other hand the current naming is more readable to most people.
There is an AppVeyor issue here to allow having badges per build environment. Currently this is only available on a per project basis (the issue has a Q21018 label)

[JamesWTruher]

really happy to broaden our CI builds!