Improve spec/views/events/show.html.erb_spec.rb

Event factory creates a weekly Sunday event. For `Event#next_occurrences`, default `start_time` is `30.minutes.ago` and `end_time` is `start_time + 10.days`. As a result, the test created one or at most two future occurrences, depending on the day of the week it was run. The proposed changes create 6 events for the test on line 28 (new 29) and make the test run reproducibly.