graph/db: add helper variable for tests

Add an isSQLDB variable that will let us quickly check in tests if the
backing DB is KVStore or SQLStore.

graph/db/test_kvdb.go

@@ -9,6 +9,9 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// isSQLDB indicates that this build does not use a SQL database.
+var isSQLDB = false
+
 // NewTestDB is a helper function that creates an BBolt database for testing.
 func NewTestDB(t testing.TB) Store {
 	backend, backendCleanup, err := kvdb.GetTestBackend(t.TempDir(), "cgr")

graph/db/test_postgres.go

@@ -11,6 +11,9 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// isSQLDB indicates that this build uses a SQL database.
+var isSQLDB = true
+
 // NewTestDB is a helper function that creates a SQLStore backed by a SQL
 // database for testing.
 func NewTestDB(t testing.TB) Store {

graph/db/test_sqlite.go

@@ -11,6 +11,9 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// isSQLDB indicates that this build uses a SQL database.
+var isSQLDB = true
+
 // NewTestDB is a helper function that creates a SQLStore backed by a SQL
 // database for testing.
 func NewTestDB(t testing.TB) Store {