Fix psqlrc test issue

[mddelk]

Add some postgres test support cleanup.

1. Pass -X when calling psql, to avoid loading user configuration.

We're using psql here to list databases, so we need clean output to parse.

This lets us avoid issues when .psqlrc files are present that result in additional stdout, such as:

\pset null '(null)'

2. Pass exception: true when running postgres system commands.

This makes errors here more obvious, e.g:

Failures:

  1) Hanami::CLI::Commands::App::DB::Create single db in app postgres creates the database
     Failure/Error: system(cmd_env, "dropdb --force #{database}", exception: true)

     RuntimeError:
       Command failed with exit 1: dropdb --force Null display is "(null)".
     # ./spec/support/postgres.rb:25:in 'Kernel#system'
     # ./spec/support/postgres.rb:25:in 'block (3 levels) in <top (required)>'
     # ./spec/support/postgres.rb:24:in 'Array#each'
     # ./spec/support/postgres.rb:24:in 'block (2 levels) in <top (required)>'