Draft: ext_external_objects: do not add conditional tests

We can't unconditionally add these tests to the test-suite, because they are optional to build. Sadly, there's no obvious way of checking the CMake condition inside the python script, so let's just revert to not adding them to the test-list at all.

This means they will have to be explicitly run instead.

Fixes: #53

TODO: replace with an MR that removes the unconditional test support instead, and just always compiles in vulkan support.