Add ability to collate per-stage reports

This adds an option, --prepare-stage-reports, which allows gitlab-qa to collect the test results (as JUnit XML files) from different scenarios and group them by Stage.

Part of https://gitlab.com/gitlab-org/quality/team-tasks/issues/316

Used by gitlab-org/quality/pipeline-common!25 (merged)

Tested using a temporary staging pipeline with a reference to the branch for gitlab-org/quality/pipeline-common!25 (merged). See https://gitlab.com/gitlab-org/quality/staging/commits/ml-test-per-stage-reports