channeldb: move makeTestDB out of test to make it available for other tests

This commit moves makeTestDB to db.go and exports it so that we'll be
able to use this function in other unit tests to make them testable with
etcd if needed.

channeldb/payments_test.go

@@ -399,7 +399,7 @@ func TestQueryPayments(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
-			db, cleanup, err := makeTestDB()
+			db, cleanup, err := MakeTestDB()
 			if err != nil {
 				t.Fatalf("unable to init db: %v", err)
 			}
@@ -512,7 +512,7 @@ func TestQueryPayments(t *testing.T) {
 // case where a specific duplicate is not found and the duplicates bucket is not
 // present when we expect it to be.
 func TestFetchPaymentWithSequenceNumber(t *testing.T) {
-	db, cleanup, err := makeTestDB()
+	db, cleanup, err := MakeTestDB()
 	require.NoError(t, err)
 
 	defer cleanup()