Top level subplots should be run in Rust too

It would be a useful part of the testing process if subplot.md and friends were also converted to be run by cargo test.

There would be a circular dependency problem if we did this at the top level however, so it makes sense to include them as part of testing subplotlib instead.

In addition, ensuring that all of runcmd.md and friends are shared properly between Python tests run by ./check and Rust tests run by cargo test would be sensible.