Fixes postgresql tests

The PostgreSQL tests were a bit of a mess. These commits clean up and fix most of the tests. It leaves three failing tests that I'll have to address later. 

```
Can_execute_stored_procedure_with_array_arguments
Can_save_complex_types_as_JSON
Can_insert_datetimeoffsets_regardless_of_current_culture
```

The first two are related to postgres-specific features - executing functions and json fields. The syntax for calling a function has changed as described in [this issue](https://github.com/npgsql/npgsql/issues/423). 

JSON columns are not playing nice because the commands are parameterized and DbType doesn't provide a JSON type. I'm having this same problem with integer arrays, but I'm still digging into it. I'm currently looking at overriding the dialect provider's `SetParameter` function to skip specifying DbType and specify NpgsqlDbType instead.