Simplify uniqueness validation spec for `IpBlock` (#32066)

spec/models/ip_block_spec.rb

@@ -3,18 +3,13 @@
 require 'rails_helper'
 
 RSpec.describe IpBlock do
-  describe 'validations' do
+  describe 'Validations' do
+    subject { Fabricate.build :ip_block }
+
     it { is_expected.to validate_presence_of(:ip) }
     it { is_expected.to validate_presence_of(:severity) }
 
-    it 'validates ip uniqueness', :aggregate_failures do
-      described_class.create!(ip: '127.0.0.1', severity: :no_access)
-
-      ip_block = described_class.new(ip: '127.0.0.1', severity: :no_access)
-
-      expect(ip_block).to_not be_valid
-      expect(ip_block).to model_have_error_on_field(:ip)
-    end
+    it { is_expected.to validate_uniqueness_of(:ip) }
   end
 
   describe '#to_log_human_identifier' do