Move mailer tests to the right file

These tests were testing mailer methods, but were inside a file for
system tests.

We're moving them now because these tests use the `open_last_email`
method, and we're looking for places where using this method might
result in a flaky test when used inside a system test.

spec/mailers/mailer_spec.rb

@@ -116,4 +116,34 @@ describe Mailer do
       end
     end
   end
+
+  describe "#machine_learning_success" do
+    let(:admin) { create(:administrator) }
+
+    it "is delivered to the user who executes the script" do
+      Mailer.machine_learning_success(admin.user).deliver
+
+      email = open_last_email
+      expect(email).to have_subject "Machine Learning - Content has been generated successfully"
+      expect(email).to have_content "Machine Learning script"
+      expect(email).to have_content "Content has been generated successfully."
+      expect(email).to have_link "Visit Machine Learning panel"
+      expect(email).to deliver_to(admin.user.email)
+    end
+  end
+
+  describe "#machine_learning_error" do
+    let(:admin) { create(:administrator) }
+
+    it "is delivered to the user who executes the script" do
+      Mailer.machine_learning_error(admin.user).deliver
+
+      email = open_last_email
+      expect(email).to have_subject "Machine Learning - An error has occurred running the script"
+      expect(email).to have_content "Machine Learning script"
+      expect(email).to have_content "An error has occurred running the Machine Learning script."
+      expect(email).to have_link "Visit Machine Learning panel"
+      expect(email).to deliver_to(admin.user.email)
+    end
+  end
 end

spec/system/admin/machine_learning_spec.rb

@@ -146,28 +146,6 @@ describe "Machine learning" do
     expect(page).to have_button "Execute script"
   end
 
-  scenario "Email content received by the user who execute the script" do
-    reset_mailer
-    Mailer.machine_learning_success(admin.user).deliver
-
-    email = open_last_email
-    expect(email).to have_subject "Machine Learning - Content has been generated successfully"
-    expect(email).to have_content "Machine Learning script"
-    expect(email).to have_content "Content has been generated successfully."
-    expect(email).to have_link "Visit Machine Learning panel"
-    expect(email).to deliver_to(admin.user.email)
-
-    reset_mailer
-    Mailer.machine_learning_error(admin.user).deliver
-
-    email = open_last_email
-    expect(email).to have_subject "Machine Learning - An error has occurred running the script"
-    expect(email).to have_content "Machine Learning script"
-    expect(email).to have_content "An error has occurred running the Machine Learning script."
-    expect(email).to have_link "Visit Machine Learning panel"
-    expect(email).to deliver_to(admin.user.email)
-  end
-
   scenario "Machine Learning visualization settings are disabled by default" do
     allow_any_instance_of(MachineLearning).to receive(:run) do
       MachineLearningJob.first.update!(finished_at: Time.current)