Add a spec for UniqueUsernameValidator (#6927)

Note that this spec has a pending test about dots in the username,
because allowing it has been reverted for now.

spec/validators/unique_username_validator_spec.rb

+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe UniqueUsernameValidator do
+  describe '#validate' do
+    it 'does not add errors if username is nil' do
+      account = double(username: nil, persisted?: false, errors: double(add: nil))
+      subject.validate(account)
+      expect(account.errors).to_not have_received(:add)
+    end
+
+    it 'does not add errors when existing one is subject itself' do
+      account = Fabricate(:account, username: 'abcdef')
+      expect(account).to be_valid
+    end
+
+    it 'adds an error when the username is already used with ignoring dots' do
+      pending 'allowing dots in username is still in development'
+      Fabricate(:account, username: 'abcd.ef')
+      account = double(username: 'ab.cdef', persisted?: false, errors: double(add: nil))
+      subject.validate(account)
+      expect(account.errors).to have_received(:add)
+    end
+
+    it 'adds an error when the username is already used with ignoring cases' do
+      Fabricate(:account, username: 'ABCdef')
+      account = double(username: 'abcDEF', persisted?: false, errors: double(add: nil))
+      subject.validate(account)
+      expect(account.errors).to have_received(:add)
+    end
+  end
+end