Add specs for UnreservedUsernameValidator (#9698)

* Add specs for UnreservedUsernameValidator

* Use instance variable

spec/validators/unreserved_username_validator_spec.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe UnreservedUsernameValidator, type: :validator do
+  describe '#validate' do
+    before do
+      allow(validator).to receive(:reserved_username?) { reserved_username }
+      validator.validate(account)
+    end
+
+    let(:validator) { described_class.new }
+    let(:account)   { double(username: username, errors: errors) }
+    let(:errors )   { double(add: nil) }
+
+    context '@username.nil?' do
+      let(:username)  { nil }
+
+      it 'not calls errors.add' do
+        expect(errors).not_to have_received(:add).with(:username, any_args)
+      end
+    end
+
+    context '!@username.nil?' do
+      let(:username)  { '' }
+
+      context 'reserved_username?' do
+        let(:reserved_username) { true }
+
+        it 'calls erros.add' do
+          expect(errors).to have_received(:add).with(:username, I18n.t('accounts.reserved_username'))
+        end
+      end
+
+      context '!reserved_username?' do
+        let(:reserved_username) { false }
+
+        it 'not calls erros.add' do
+          expect(errors).not_to have_received(:add).with(:username, any_args)
+        end
+      end
+    end
+  end
+end