Change language link download tests to use parameterized links

This is an attempt to address the timeout issue that we are having with `test_links_on_firefox_organization_all`. There are so many links to be tested that the test is taking too long and timing out. We could just test a subset of links, but that would mean losing coverage. This PR changes it so that each link will be tested in a separate test which should eliminate this timeout issue.

I have used a technique that was proposed by @davehunt in a pull request against qmo-tests [1]. This is not identical to Dave's code, as it is not generic and is specific to just these language link tests, but if we like it in this case we could consider using it for other link-checking tests as well.

[1] https://github.com/mozilla/qmo-tests/pull/57