Combine test report metrics with childrens reports

[jasonwliu]

• Adds new option for gathering children test reports
• Additional tests for pipeline test reports
• Includes fixes in https://github.com/mvisonneau/gitlab-ci-pipelines-exporter/pull/670

Closes https://github.com/mvisonneau/gitlab-ci-pipelines-exporter/issues/669 and https://github.com/mvisonneau/gitlab-ci-pipelines-exporter/issues/671.

[coveralls]

coverage: 75.446% (+0.1%) from 75.317% when pulling d07e1957f54a55e4cf27f06a1f1736d656709179 on jasonwliu:children-test-reports into a894122a8cb2c8702f723dcfba416c72673ca82b on mvisonneau:main.

[mvisonneau]

thanks @jasonwliu !