channeldb+htlcswitch: don't use fastsha256 in tests

The btcsuite/fastsha256 registers itself in the crypto package of
golang as a replacement for sha256. This causes problems in TLS1.3
connections that require the hash implementations to be serializable
and results in the "tls: internal error: failed to clone hash" error.
By removing all uses of the library we fix that error.

go.mod

@@ -14,7 +14,6 @@ require (
 	github.com/btcsuite/btcwallet/wallet/txrules v1.0.0
 	github.com/btcsuite/btcwallet/walletdb v1.3.1
 	github.com/btcsuite/btcwallet/wtxmgr v1.1.1-0.20200515224913-e0e62245ecbe
-	github.com/btcsuite/fastsha256 v0.0.0-20160815193821-637e65642941
 	github.com/davecgh/go-spew v1.1.1
 	github.com/go-errors/errors v1.0.1
 	github.com/go-openapi/strfmt v0.19.5 // indirect