[BuildStream] Remote-execution testing

[Martin Blanchard <tchaik gmx com>]

Hello,

With remote-execution support now being part of the BuildStream 2.0
plan [1], improving BuildStream's remote-execution client code coverage
and easing the complexity of spinning up a remote-execution service for
local testing has become crucial. Below is an list of recent
improvements that have been made toward these goals.

* Systematic testing in CI:

  A dedicated test job has recently been added to the CI pipeline:
  'tests-remote-execution' [2]. It runs remote builds of the simple
  autotools project and uses BuildGrid [3] as remote-execution
  service. BuildGrid hasn't had any stable release yet, so that CI job
  relies on nightly images. Considering that there are not stability
  guaranties using those images, the CI job is allow to fail, thus not
  blocking a merge.

  This new CI job exercises the _sandboxremote.py code and integrates
  coverage results in the main coverage report. In order to make that
  report easier to access and read, we are now publishing it as static
  HTML pages (for master CI pipelines) [4].

* Manual testing:

  For further experiments with remote-execution, we now provide custom
  Docker Compose manifests along with usage instructions at the
  BuildGrid repository dedicated to  building nightly images for this
  remote-execution service [5]. Spinning up a BuildGrid instance should
  be as simple as invoking 'docker-compose up'.

* New remote-execution label:

  A new 'Remote Execution' GitLab label has been created recently and
  existing issues have been tagged. This helps us keeping track of
  remote-execution related bugs and enhancement requests. Please use
  it! Tagged issues can be queried by clicking on that label [6].

Cheers,
Martin

[1]
https://buildstream.build/articles/2019/buildstream-news-and-2.0-planning/
[2] https://gitlab.com/BuildStream/buildstream/merge_requests/1239
[3] https://gitlab.com/BuildGrid/buildgrid
[4] https://buildstream.gitlab.io/buildstream/coverage/coverage-report/
[5] https://gitlab.com/BuildGrid/buildgrid.hub.docker.com
[6]
https://gitlab.com/BuildStream/buildstream/issues?label_name%5B%5D=Remote+Execution