Testing different site SXL versions

[emiltin]

Running on a matrix of core version is possible after #422, and is used for the gem tlc and supervisor tests.

Testing sites with differnt SXL version is not easy since we can't control the SXL from the supervisor (validator) side.

One option might be to just report results for the SXL version that the site uses. The site can then use differnt SXL version on different runs, to build up a set of results for different SXL versions. If we want to to this route, perhaps we should think of increasing the test frequence. e.g. to hourly. In any case it could be a benefit to run more frequently just to get more data.

[emiltin]

Ideas:

• we modify the Version message so that the site sends a list of supported SXL version, and the supervisor chooses the version and returns it in the Version message it sends. (similar to how the core version is negotiated.)
• the site uses a picks supported SXLs at random when connecting, over time results for all SXLs are collected.
• the supervisor could reject the initial Version message if the SXL is not what it would like to test. The rejection could mention the desired SXL. The site would then re-connect with the desired SXL.