[docs-infra] Test the link check script

[alexfauquette]

          The cleanest way I can think of would be to extract the script logic into packages-internal/scripts, test it there and leave just the CLI in the scripts directory.

Originally posted by @michaldudak in #43144 (comment)

For context, the script which takes care of verifying each internal link point to an existing page, and an existing anchor was doing nothing (except eating some CPU time)

It would be nice to have a test that ensures it fails

The idea would be to have two markdown files, one with broken links, one with ok links, and run test that ensure the script is failing on the correct one and passing the other one.

This should avoid the issue we got with the script silently failing because it detected no link

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[DiegoAndai]

Hey @alexfauquette, what needs to be fixed specifically?

[alexfauquette]

This issue is about testing the script is working as expected.

The idea would be to have two markdown files, one with broken links, one with ok links, and run test that ensure the script is failing on the correct one and passing the other one.

This should avoid the issue we got with the script silently failing because it detected no link