test view: don't create links for missing builds

[khilman]

In the tests view, before making the build name into a clickable link, check to see if build exists.

Downstream users can use KCI to show results for tests that do not use KCI builds, so don't create link to for builds which are not present.

[gctucker]

I was just talking about that with @alexandrasp earlier today.

Doing the HTTP equivalent of curl -I <URL on the frontend before creating links from the storage server should work fine. I guess we also need to work out what is the best way to show to the user that a file is not present. Just not having a link but only the text is one option, but it might look a bit like a bug compared to other views that have the link. Maybe crossing out the text would make it clearer that it's not available i.e. Image or modules.tar.xz.

[gctucker]

Also, kernelci.org should at some point start showing test results that don't involve a build - things like DT validation, KUnit or static analysis. So this will apply not just to downstream users.