Add specs for AdminMailer (#9597)

spec/mailers/admin_mailer_spec.rb

+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe AdminMailer, type: :mailer do
+  describe '.new_report' do
+    let(:sender)    { Fabricate(:account, username: 'John', user: Fabricate(:user)) }
+    let(:recipient) { Fabricate(:account, username: 'Mike', user: Fabricate(:user, locale: :en)) }
+    let(:report)    { Fabricate(:report, account: sender, target_account: recipient) }
+    let(:mail)      { described_class.new_report(recipient, report) }
+
+    it 'renders the headers' do
+      expect(mail.subject).to eq("New report for cb6e6126.ngrok.io (##{report.id})")
+      expect(mail.to).to eq [recipient.user_email]
+      expect(mail.from).to eq ['notifications@localhost']
+    end
+
+    it 'renders the body' do
+      expect(mail.body.encoded).to eq("Mike,\r\n\r\nJohn has reported Mike\r\n\r\nView: https://cb6e6126.ngrok.io/admin/reports/#{report.id}\r\n")
+    end
+  end
+end