Move non-empty files from test dir to spec and adopt specs

Signed-off-by: Sebastian Serth <Sebastian.Serth@student.hpi.de>

spec/mailers/previews/user_mailer_preview.rb

@@ -0,0 +1,7 @@
+class UserMailerPreview < ActionMailer::Preview
+  def exercise_anomaly_detected()
+    collection = ExerciseCollection.new(name: 'Hello World', user: FactoryBot.create(:admin))
+    anomalies = {49 => 879.325828, 51 => 924.870057, 31 => 1031.21233, 69 => 2159.182116}
+    UserMailer.exercise_anomaly_detected(collection, anomalies)
+  end
+end