link: increase expiry grace delta

This commit increase the expiry grace delta to a value above the
broadcast delta. This prevents htlcs from being accepted that would
immediately trigger a channel force close.

A correct delta is generated in server.go where there is access to
the broadcast delta and passed via the peer to the links.

Co-authored-by: Jim Posen <jim.posen@gmail.com>

htlcswitch/test_utils.go

@@ -1019,6 +1019,7 @@ func (h *hopNetwork) createChannelLink(server, peer *mockServer,
 		fwdPkgTimeout       = 15 * time.Second
 		minFeeUpdateTimeout = 30 * time.Minute
 		maxFeeUpdateTimeout = 40 * time.Minute
+		expiryGraceDelta    = 3
 	)
 
 	link := NewChannelLink(
@@ -1048,6 +1049,7 @@ func (h *hopNetwork) createChannelLink(server, peer *mockServer,
 			MinFeeUpdateTimeout: minFeeUpdateTimeout,
 			MaxFeeUpdateTimeout: maxFeeUpdateTimeout,
 			OnChannelFailure:    func(lnwire.ChannelID, lnwire.ShortChannelID, LinkFailureError) {},
+			ExpiryGraceDelta:    expiryGraceDelta,
 		},
 		channel,
 	)