*/releases: Specify if a release should be tested or not

Each release line is now in the following format:
`release[*][|other,key,words]`

 - * indicates that the release is unsupported
 - keywords are comma-separated, and can currently be `test` or
   `notest`. Default is for supported releases to be tested (can
   be overridden with `notest`), and unsupported ones not to be
   tested (can be overridden with `test`).