adding fix for flaky test

[shanbady]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/8387

Description (What does it do?)

This PR fixes a flaky test which fails when learning resources have the same platform and readable id which violates a db constraint

How can this be tested?

1. checkout main
2. in the web container install pytest-repeat pip install pytest-repeat
3. run the flaky test enough times to see it fail pytest learning_resources/tasks_test.py::test_remove_duplicate_resources --count 100 -x
4. checkout this branch
5. re-run the test and see it pass 100% of the time

[mbertrand]

👍