Streamlined way of specifying and forcing a `platform`

[ijpulidos]

While dealing with issue #993 I realized that the platform gets changed along the way for some of the tests. In general, I think this is something that needs improvement. We need a better way to specify a platform (related to #939 ), but not only test that the platform is available but to actually force the complete workflow to use the specified platform. This is required to also get the expected accuracy when validating energy components or similar.