pickfirst: Don't use internal.ShuffleAddressListForTesting directly

Based on the PR review comment https://github.com/grpc/grpc-go/pull/7498#discussion_r1718899646,

pickfirst sets ShuffleAddressListForTesting to use rand.Shuffle in its init function
pickfirst also directly uses ShuffleAddressListForTesting in UpdateClientConnState to get the actual shuffle function

So, for someone reading the code, it can be a little misleading as to why pickfirst is getting the shuffle func from a var with a ForTesting suffix.

The suggestion from the comment is to instead do something like:

package pickfirst

var shuffleFunc func(n int, swap func(i, j int))

func init() {
  shuffleFunc = rand.Shuffle
  internal.ShuffleAddressListForTesting = func(sf func(n int, swap func(i, j int))) {
    shuffleFunc = sf
  }  
}

func (...) UpdateClientConnState(...) {
  // Directly use `shuffleFunc` to do the shuffling
}

Note that I've changed the signature of internal.ShuffleAddressListForTesting to accept a func. We could rename it to something like internal.PickfirstShuffleFunc. Also, note that we could completely move this var to the pickfirst package and have our tests directly use that, but would pollute the external API.

grpc / grpc-go

pickfirst: Don't use internal.ShuffleAddressListForTesting directly #7515