sqldb+invoices: create a re-usable comparion helper function

So that we can use it for comparisons in other migrations too.

go.mod

@@ -140,7 +140,7 @@ require (
 	github.com/opencontainers/image-spec v1.0.2 // indirect
 	github.com/opencontainers/runc v1.1.12 // indirect
 	github.com/ory/dockertest/v3 v3.10.0 // indirect
-	github.com/pmezard/go-difflib v1.0.0
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/prometheus/client_model v0.2.0 // indirect
 	github.com/prometheus/common v0.26.0 // indirect
 	github.com/prometheus/procfs v0.6.0 // indirect