utils/test-pkg: add gitlab-ci support

utils/test-pkg: add gitlab-ci support

The gitlab-ci support in test-pkg allows to parallelize the test-pkg
work into several gitlab jobs. It's much faster than local serialized
testing.

To trigger this, a developer will have to add, in the latest commit of
their branch, a token on its own line, followed by a configuration
fragment, e.g.:

    test-pkg config:
    SOME_OPTION=y
    # OTHER_OPTION is not set
    SOME_VARIABLE="some value"

This configuration fragment is used as input to test-pkg.

To be able to generate one job per test to run, we need the list of
tests in the parent pipeline, and the individual .config files (one per
test) in the child pipeline. We use the newly-introduced --prepare-only
mode to test-pkg, and collect all the generated .config files as
artefacts; those are inherited in the child pipeline via the
"needs::pipeline" and "needs::job" directives. This is a bit tricky,
and is best described by the Gitlab-CI documentation [0].

We also list those .config files to generate the actual list of jobs to
run in the child pipeline.

Notes:
  - if the user provides an empty fragment, this is considered an error:
    indeed, without a fragment (and the package name), there is no way
    to know what to test;
  - if that fragment yields an empty list of tests, then there is
    nothing to test either, so that is also considered an error.

[0] https://docs.gitlab.com/ee/ci/yaml/README.html#artifact-downloads-to-child-pipelines

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[yann.morin.1998@free.fr:
  - split the change to test-pkg to its own patch
  - generate the actual yml snippet in support/scripts/generate-gitlab-ci-yml,
    listing the .config files created by test-pkg
  - some code-style-candies...
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>