MQE-1256: "Merge" works different for tests and action groups

[KevinBKozan]

Description

• Actiongroup and test dom classes now check for duplicate child and elements.
• Minor bugfix around searching filename, "test" is literally always going to appear in filepath due to directory structure.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/verification tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)
• Changes to Framework doesn't have backward incompatible changes for tests or have related Pull Request with fixes to tests

[coveralls]

Coverage increased (+0.05%) to 57.524% when pulling 53a0f69 on MQE-1256-2 into de1ab89 on develop.