How to compare popularity of MediaWiki (eg. Wikipedia) pages

Well, I am not sure you really need that. I do something similar and at the beginning I was also looking for a way to create the back relation but now I do not thing I need that.

You just need 2 pages, one for "Testcase XYZ" and one for "Requirement 12345".

In your page Testcase XYZ you already listed all requirements associated to you test case. So now you just need in your page "Requirement 12345" to see all Testcases related to your Requirement 12345:

{{#ask:format="table"|[[Category:Test case] |?RequirementID}}

Is this way not a solution for you?

You can use CatScan to do that.

For example, to query files on Commons, you can start with http://toolserver.org/~magnus/catscan_rewrite.php?language=commons&project=wikimedia&ns[6]=1 and enter the categories you want into the Categories box.