Make the CI more generic

Emil Velikov requested to merge xexaxo/virglrenderer:ci-generic-test into master

A lengthy, mixed bag, series for the CI:

  • updates apt-get handling to the latest Docker recommendations
  • paves towards unified test suite - --test-list
  • enables parallel jobs in piglit
  • misc cleanups
  • folds the unstable test execution alongside the must-pass ones
  • the tests use a generic fragment/template

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