MQE-970: Tests in Suites being omitted when line counts are exact match

[KevinBKozan]

Description

• array_diff to array_diff_key in ParallelGroupSorter.

Fixed Issues (if relevant)

1. magento/magento2-functional-testing-framework#970: Tests in Suites being omitted when line counts are exact match

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 remained the same at 50.338% when pulling cfe1aca on MQE-970 into d7b3ce6 on develop.