sweepbatcher: fix TestPresigned/dust_main_output

The Problem:
The test TestPresigned/dust_main_output was causing a deadlock. When the test
called batcher.AddSweep for the second time (with a valid sweep), the batcher
would create a new batch and attempt to register for a spend notification.
However, the test was not waiting for or consuming this notification
registration, leading to a blocked goroutine and a timeout.

The Fix:
I have added a line to the test to consume the spend notification registration
from the mock lnd's channel, similar to how other tests in the same file are
structured. This prevents the deadlock.

The problem was introduced in commit 54652dc.

Author: starius

sweepbatcher/sweep_batcher_presigned_test.go

@@ -1503,6 +1503,10 @@ func testPresigned_presigned_group_with_dust_main_output(t *testing.T,
 		Notifier: &dummyNotifier,
 	})
 	require.NoError(t, err)
+
+	// Since a batch was created we check that it registered for its primary
+	// sweep's spend.
+	<-lnd.RegisterSpendChannel
 }
 
 // testPresigned_presigned_group_with_dust_below_relay_fee passes a tx with a