Test many bitcoind instances

[RCasatta]

Investigation for https://github.com/lightningdevkit/rust-lightning/issues/2438

I can reproduce locally a similar error as mentioned in the issue with a big amount of processes like in the included test. It seems there is a Transport failure in the create_wallet, consequently causing the failure of load_wallet. The included commit passes the added tests but doesn't fix it, it just considers ok if the default wallet is not created. In another branch I tried to repeat the create_wallet but the error seems unrecoverable.

Not sure how to properly fix, maybe adding a Config flag to avoid the creation of the default wallet, delegating it to clients is an option