SPV: update testnet seed nodes

[Bushstar]

Update the SPV DNS seeds for testnet as all bar one of the existing ones were non-responsive.

[prasannavl]

Running all them through dig.

Both old and new *.petertodd.org seems to the most unreliable, while testnet-seed.breadwallet.com seems to work quite well across many different regions.

$ dig seed.tbtc.petertodd.org

; <<>> DiG 9.18.4-2ubuntu2-Ubuntu <<>> seed.tbtc.petertodd.org
;; global options: +cmd
;; connection timed out; no servers could be reached

Perhaps, we should remove petertodd ones for the moment as workaround until we can address the bread wallet connection code to be more graceful and resilient to unreliable domains.