Self test description question

[hamish]

A PID SUPPORTED_SELFTESTS would in this case be in lieu of SELFTEST_COUNT so no real difference in overhead.
I make the assumption that any responder supporting many self tests will not see an extra byte of code space for each test as a big overhead, so here I buy into the cardinal approach.

There may be a slight impact on the controller side, but in any case, less than the current arrangement demands.

I make the assumption that we're all in favour of making support for SELFTEST_DESCRIPTION mandatory when PERFORM_SELFTEST is supported.