Use the Electron Packager official platform/arch list

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

This replaces the broken test on Windows with a set of unit tests that just tests parsing an arch value. Retrieving archs when arch is all has been delegated to Electron Packager.

This is a WIP because I want to make sure the testsuite passes in AppVeyor.

TODO

• Add API to Electron Packager to get arches for a given platform and version (needed for the arm64 + Linux + Electron >= 1.8.0 case) - Add target arch API packager#727
• Utilize the new API

[MarshallOfSound]

As long as we're all clear that electron-packager/targets is now public API I'm 👍

[malept]

I should probably document it more but anything exported in Packager is fair game as far as "public API" is concerned. If it's exported, more often than not I have unit tests for it.

I just need to remember to major version bump more often.