[MNG-7002] Added a test scenario for including child modules when usi…

[MartinKanters]

…ng the --pl switch.

This was easy to pick up since MNG-6118 extended the testcase with a multi-level hierarchy project:
https://github.com/apache/maven/pull/373/files#diff-72ccd31ee50f74b4685bab505c4fbbe21f1eea8c55aea23c75ad9b620a462c06R73-R80

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[jira-importer]

Resolve #8029