Makefile outdated?

The makefile has commands for per-backend testing, e.g.

```
test-postgresql: reset-pgsql
stack test esqueleto:postgresql
```

But if you actually run

```
stack test esqueleto:postgresql
```

You get:

```
Error parsing targets: Component postgresql does not exist in package esqueleto
```

I'm guessing this is an old way of running the tests. I'm not sure if running single database backend is actually possible anymore? I might just be not finding where that's configured though

Start by reading the Makefile targets such as test-postgresql and compare them with the documented `stack test esqueleto:postgresql` command. Check whether single-database-backend testing is still supported, then update or remove the outdated target so the Makefile reflects the current test workflow.

Written by the indexing model from the issue text.