rpc, internal/guide: speed up tests a bit

[holiman]

This should remove about two minutes 75s on CI.

The internal/guide test was news to me, but apparently it executes the commands on the page here: https://geth.ethereum.org/docs/dapp/native-accounts . Well, doing full scrypt on CI is very slow, so maybe we don't have to do it all the time.

On Image: Visual Studio 2019; Environment: GETH_ARCH=386, GETH_MINGW=C:\msys64\mingw32, without this PR

ok  	github.com/ethereum/go-ethereum/internal/guide	18.755s	coverage: [no statements]
...
ok  	github.com/ethereum/go-ethereum/rpc	107.314s	coverage: 84.1% of statements

With this PR

ok  	github.com/ethereum/go-ethereum/internal/guide	1.751s	coverage: [no statements]
...
ok  	github.com/ethereum/go-ethereum/rpc	49.667s	coverage: 83.7% of statements

[holiman]

These are the biggest timesinks on the windows CI

package	time
github.com/ethereum/go-ethereum/tests	320.869
github.com/ethereum/go-ethereum/eth/downloader	150.764
github.com/ethereum/go-ethereum/p2p/discover	99.935
github.com/ethereum/go-ethereum/trie	82.508
github.com/ethereum/go-ethereum/consensus/ethash	77.82
github.com/ethereum/go-ethereum/les	69.552
github.com/ethereum/go-ethereum/eth/protocols/snap	62.451
github.com/ethereum/go-ethereum/les/downloader	58.636
github.com/ethereum/go-ethereum/core	58.187
github.com/ethereum/go-ethereum/rpc	49.667
github.com/ethereum/go-ethereum/eth/fetcher	44.512
github.com/ethereum/go-ethereum/les/fetcher	40.346
github.com/ethereum/go-ethereum/core/vm	36.163
github.com/ethereum/go-ethereum/cmd/devp2p/internal/ethtest	34.392
github.com/ethereum/go-ethereum/les/vflux/server	34.363
github.com/ethereum/go-ethereum/cmd/geth	23.341
github.com/ethereum/go-ethereum/core/state	19.69
github.com/ethereum/go-ethereum/core/txpool	17.696
github.com/ethereum/go-ethereum/les/flowcontrol	17.407
github.com/ethereum/go-ethereum/metrics	16.932
github.com/ethereum/go-ethereum/core/rawdb	16.279
github.com/ethereum/go-ethereum/miner	15.707
github.com/ethereum/go-ethereum/eth	14.822
github.com/ethereum/go-ethereum/crypto/bls12381	13.906
github.com/ethereum/go-ethereum/accounts/keystore	13.885