[BuildStream] WSL pre-merge tests have been added to the CI

[Jonathan Maw <jonathan maw codethink co uk>]

Hi,

The more eagle-eyed may have noticed that there is now a WSL job in 
buildstream's CI.
These tests intend to run through everything that is doable in WSL as it 
currently is and catch anything that breaks on WSL.

WSL is currently incapable of running any tests that require a sandbox 
(the current workflow for WSL users is to start jobs over buildgrid), so 
any users who add tests that use a sandbox should import the 
`HAVE_SANDBOX` symbol from `testutils.site` and decorate those tests 
with `@pytest.mark.skipif(not HAVE_SANDBOX)`.

There is a limited number of gitlab-runners for WSL, currently I have 
one machine dedicated to being a gitlab-runner, and another spare for 
the moment, so let me know if it becomes a bottleneck on getting things 
merged.

For those who are interested, a VM for the gitlab-runners can be 
generated automatically using scripts (based on some provided by Jürg) 
in https://gitlab.com/jonathanmaw/wsl-setup-script.

Best regards,

Jonathan

--
Jonathan Maw, Software Engineer, Codethink Ltd.
Codethink privacy policy: https://www.codethink.co.uk/privacy.html