Start with tidier RPC-test cache directories

Remove more files from the cached, 200-block-chain data directories.
An old peers.dat caused me confusion when testing the -regtest-uses-.onion-seeds bug.

@laanwj : this is the cache/ directory setup code, it starts up nodes just for the purpose of creating an initial blockchain (so tests don't take so long to run). If rpc tests start their own nodes with -disablewallet, it won't affect this code.