Replace DISABLE env variables with ENABLE ones

Having variables in the code with negative names is counter intuitive, particularly if we ever need to negate them. Also, it seems that we are going to have jobs that are disabled by default as well as the ones that are enabled by default.

So, I would like to replace the DISABLE variables with ENABLE ones, we could also add a deprecation message if we find the DISABLE variables set, this means adding the full list of test variables in the top of the file, that should make it easier to see which tests are to be run.