[melodic] Use test utils in acceptance_test_blending.py

[martiniil]

Description

Just removing some duplicate code. (backport)

Things to add, update or check by the maintainers before this PR can be merged.

• [x] Public api function documentation
• [x] Architecture documentation reflects actual code structure
• [x] Tutorials
• [x] Overview on ROS wiki
• [x] Package Readme (example pilz_robots)
• [x] Good commit messages (some tips)
• [x] CHANGELOG.rst updated
• [x] Copyright headers
• [x] Examples

Review Checklist

• [ ] Soft- and hardware architecture (diagrams and description)
• [ ] Test review (test plan and individual test cases)
• [ ] Documentation describes purpose of file(s) and responsibilities
• [ ] Code (coding rules, style guide)

Release planning (please answer)

• [x] When is the new feature released? Does not matter.
• [x] Which dependent packages do have to be released simultaneously? None.