Add CI for native windows support

Background

NOTE: This is based on the assumption that win32 refers to the windows API, not running on a 32-bit Windows system, as there is some ambiguity.

There is an ongoing effort (#1021) to add native windows support to buildstream, and a need to ensure that this support doesn't bit rot, or at least where functionality breaks it is known exactly when this happens.

The proposed solution is to set up a CI runner and CI jobs to exercise the currently-implemented functionality. Work has apparently been made to support:

• bst init
• bst help
• bst workspace list (possibly)

And there are currently experimental branches to allow bst build for import elements.

Task description

• Manually set up a Windows system to run CI jobs in native Windows
  • Install Python and Pip
  • Install BuildStream's pip dependencies
  • Install a native windows gitlab runner
• Produce automated instructions to spin up a runner from scratch
  • There is already a script to spin up a runner at gitlab.com/jonathanmaw/wsl-setup-script that can be modified, though if there's spare time it might be worth looking into other tools (ansible or packer scripts?)
• Add gitlab CI for native windows to exercise bst init and `bst help functionality.
  • Using tox or pytest is not necessary, but will be simpler if tox or pytest "just work" (modulo nearly every test failing).
  • If these steps prove to be time-consuming, these tests should be set to only run on master, otherwise we will block buildstream's CI.
• Install the experimental branches of buildbox-common and buildbox-casd in native windows.
  • Building these is likely to be highly time-consuming.
• Produce automated instructions for buildbox-common and buildbox-casd installation.
  • It must be relatively easy to specify different refs of buildbox-common and buildbox-casd, so upgrading to the latest version of these (e.g. when there are more fixes) is easy.
• Add gitlab CI for native windows to exercise bst build on import elements.
  • It must be relatively easy to disable this or make it optional.

Acceptance Criteria

• When CI runs against master, tests are run to check windows 10 support for:
  • bst init
  • bst help
  • bst build of an import element.
• The test for bst building an import element can be easily disabled.

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