test: make mail checks in system specs less flakey (#765)

These specs currently fail semi-frequently since there is no actual
guarantee that the emails will be in the same order every time, so I've
rewritten the tests to select all emails with the expected `to` address

Author: G-Rath

spec/system/approved_user_application_spec.rb

@@ -21,17 +21,21 @@
   it "sends an email to the user after submitting" do
     subject.fill_in_mandatory_fields
     perform_enqueued_jobs { subject.submit }
-    mail = ActionMailer::Base.deliveries[-2]
-    expect(mail.to).to eq [user.email]
-    expect(mail.subject).to eq I18n.t("approved_user_mailer.submitted.subject")
+
+    mails = ActionMailer::Base.deliveries.select { |mail| mail.to.include?(user.email) }
+
+    expect(mails.length).to be(1)
+    expect(mails.first.subject).to eq I18n.t("approved_user_mailer.submitted.subject")
   end
 
   it "sends an email to the admin after submitting" do
     subject.fill_in_mandatory_fields
     perform_enqueued_jobs { subject.submit }
-    mail = ActionMailer::Base.deliveries[-1]
-    expect(mail.to).to eq [admin.email]
-    expect(mail.subject).to eq I18n.t("approved_user_mailer.admin_submitted.subject")
+
+    mails = ActionMailer::Base.deliveries.select { |mail| mail.to.include?(admin.email) }
+
+    expect(mails.length).to be(1)
+    expect(mails.first.subject).to eq I18n.t("approved_user_mailer.admin_submitted.subject")
   end
 
   it "leaves a required field blank" do