Decouple test definition templates from the config object attributes

Note: I wonder if it might actually be better to ultimately decouple the templates from the config object attributes, so just having `{{ platform_type }}` would work better, and each Runtime implementation can populate accordingly. But that's a bit beyond the scope of this PR.

_Originally posted by @gctucker in https://github.com/kernelci/kernelci-core/pull/2153#discussion_r1379016807_

Start with the linked discussion in pull request 2153, which raises decoupling test definition templates from config object attributes. Clarify how Runtime implementations should populate template values and define the expected behavior for direct values such as `{{ platform_type }}`; the issue does not name files or tests.

Written by the indexing model from the issue text.