Add test for detecting broken and locked links

[mfarhan943]

Description

This PR reintroduces a previously reverted test case for detecting broken and locked links. It mocks _validate_urls_access_in_batches to return a list containing broken (400) and locked (403) URLs.

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Supporting information

Reverted PR: #36238

Testing instructions

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[rayzhou-bit]

lgtm!

[hinakhadim]

I've tested 5 times and the test has passed all times for me.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.