Test daisy chained homogeneous spike instances. (#334)

* Test debugging multiple spikes in a daisy chain.

* Hugely speed up rbb_daisychain.

Now 2 dual-hart spikes are less than 4x slower than a single dual-hart
spike.

* WIP

* Test daisy chained homogeneous spike instances.

For OpenOCD, this means we're checking that we can talk to multiple
TAPs. Next up is heterogeneous testing.

* Enable Sv48Test.

Didn't mean to disable it with this commit.

* Test authentication again.

Another change I hadn't meant to push...