Change testing quorum size

[jagerman]

Currently our testing quorum size is max{1%, 50}.

On mainnet the percentage here is probably too low; 50 currently is just under 3%, and testing intervals already seem a bit on the long side.

On testing this is a real pain because when testing some new thing that causes decommissions it is fairly common to get a single-block that decomms a whole bunch of nodes and leaves the network stalled (in mining-required mode) because there are too few active nodes left.

I suggest we change it to max{3%, 2}, which will keep mainnet about where it is (e.g. 54 tests for 1800 nodes), and will significantly rate-limit testnet's ability to self-destruct to 2 decomms per block.

[jagerman]

(NB: this is a consensus-breaking change)