[BuildStream] BuildStream overnight tests

[Tiago Gomes <tiago gomes codethink co uk>]

Hi all,

Recently, a set of overnight tests [1] were added to BuildStream.

These tests attempt to build a known ref of the freedesktop-sdk with 
their cache server and without using the cache server, for the master 
and 1.2 branches.

Obviously, for these tests to be useful, we should regularly keep an eye 
on the CI results: 
https://gitlab.com/BuildStream/buildstream/pipeline_schedules .