htlcswitch: clean up test resources and temporary files

The htlcswitch tests were creating temporary database files
but failing to clean them up.

We fix this by making it obvious when temporary db files are
created, and cleaning up those resources where necessary in
the tests.

htlcswitch/link_test.go

@@ -1945,7 +1945,8 @@ func newSingleLinkTestHarness(chanAmt, chanReserve btcutil.Amount) (
 
 	cleanUp := func() {
 		close(alicePeer.quit)
-		defer fCleanUp()
+		invoiceRegistry.cleanup()
+		fCleanUp()
 	}
 
 	return aliceLink, bobLc.channel, bticker.Force, start, cleanUp,