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.sum

@@ -52,8 +52,6 @@ github.com/btcsuite/btcwallet/wtxmgr v1.0.0 h1:aIHgViEmZmZfe0tQQqF1xyd2qBqFWxX5v
 github.com/btcsuite/btcwallet/wtxmgr v1.0.0/go.mod h1:vc4gBprll6BP0UJ+AIGDaySoc7MdAmZf8kelfNb8CFY=
 github.com/btcsuite/btcwallet/wtxmgr v1.1.1-0.20200515224913-e0e62245ecbe h1:yQbJVYfsKbdqDQNLxd4hhiLSiMkIygefW5mSHMsdKpc=
 github.com/btcsuite/btcwallet/wtxmgr v1.1.1-0.20200515224913-e0e62245ecbe/go.mod h1:OwC0W0HhUszbWdvJvH6xvgabKSJ0lXl11YbmmqF9YXQ=
-github.com/btcsuite/fastsha256 v0.0.0-20160815193821-637e65642941 h1:kij1x2aL7VE6gtx8KMIt8PGPgI5GV9LgtHFG5KaEMPY=
-github.com/btcsuite/fastsha256 v0.0.0-20160815193821-637e65642941/go.mod h1:QcFA8DZHtuIAdYKCq/BzELOaznRsCvwf4zTPmaYwaig=
 github.com/btcsuite/go-socks v0.0.0-20170105172521-4720035b7bfd h1:R/opQEbFEy9JGkIguV40SvRY1uliPX8ifOvi6ICsFCw=
 github.com/btcsuite/go-socks v0.0.0-20170105172521-4720035b7bfd/go.mod h1:HHNXQzUsZCxOoE+CPiyCTO6x34Zs86zZUiwtpXoGdtg=
 github.com/btcsuite/golangcrypto v0.0.0-20150304025918-53f62d9b43e8 h1:nOsAWScwueMVk/VLm/dvQQD7DuanyvAUb6B3P3eT274=