Added tests for FutureGetBestBlockHashResult.Receive

[ClaytonNorthey92]

Adding a few tests to cover func (f FutureGetBestBlockHashResult) Receive(). I've noticed that rpcclient has 0.6% test coverage (before this PR):

$ git log -1
commit ad26585576912588d213dbe973b222fcec621546 (HEAD, origin/master, origin/HEAD)
Author: David Cardenas <47157243+davidcardenasus@users.noreply.github.com>
Date:   Tue Oct 17 02:47:09 2023 -0700

$ go-acc ./rpcclient
ok  	github.com/btcsuite/btcd/rpcclient	0.148s	coverage: 0.6% of statements in ./rpcclient

this PR bumps it up to 0.9%

$ go-acc ./rpcclient
ok  	github.com/btcsuite/btcd/rpcclient	0.234s	coverage: 0.9% of statements in ./rpcclient

I want to keep adding tests but I want to do them in their own PRs: as some tests will be larger than others, I don't want the PR(s) to be too big and unreadable.

[jcvernaleo]

One test per PR to keep them small makes perfect sense to me.

[jcvernaleo]

OK