Get validates_uniqueness_of to work better with `NOT NULL` columns

Instead of having to persist the subject, like so:

```ruby
  describe 'validations' do
    subject { build(:model) }
    it { validates_presence_of :required_attr }
    context "persisted" do
      # Needs to be persisted to prevent NOT NULL db error
      subject { create(:model) }
      it { validates_uniqueness_of :unique_attr }
    end
  end
```

You can now just write:

```ruby
  describe 'validations' do
    subject { build(:model) }
    it { validates_presence_of :required_attr }
    it { validates_uniqueness_of :unique_attr }
  end
```

When required the matcher saves the record to the database.
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