lnd+sweep: move DetermineFeePerKw into config

This commit makes `DetermineFeePerKw` configurable on sweeper so it's
easier to write unit tests for it.

sweep/sweeper_test.go

@@ -149,6 +149,7 @@ func createSweeperTestContext(t *testing.T) *sweeperTestContext {
 		},
 		MaxFeeRate:        DefaultMaxFeeRate,
 		FeeRateBucketSize: DefaultFeeRateBucketSize,
+		DetermineFeePerKw: DetermineFeePerKw,
 	})
 
 	ctx.sweeper.Start()