Deleting a single parametrized test case from a test execution results in all related test cases to be deleted

[RelapsedCode]

Description of problem

Deleting a single parametrized test case from a test execution results in all related test cases to be deleted

Version or commit hash (if applicable)

Version 12.6.1

Steps to Reproduce

1. Create a parametrized test case with more than one value for a parameter
2. Add the test case into a test plan
3. Create a new test execution from the test plan
4. Select only one test case and delete it

Actual results

All derivative test cases are deleted

Expected results

Only the selected test case should be deleted

[atodorov]

Initial work in https://github.com/kiwitcms/Kiwi/pull/3282 but needs more work.