Fix tests flakyness

- Correct the bounds for end time and start time between lesson and
  section based on the related lesson
- Add sequence to the name of the instrument to skip collisions with
  fixtures. System test rely on the fixtures so for now this should
  suffice.

spec/factories/instrument.rb

@@ -1,5 +1,5 @@
 FactoryBot.define do
   factory :instrument do
-    name { Faker::Music.instrument }
+    sequence(:name) { |n| "#{Faker::Music.instrument}-#{n}" }
   end
 end

spec/factories/lesson.rb

@@ -5,7 +5,7 @@
     end
     video_url { Faker::Internet.url(host: "youtu.be", scheme: "https") }
     duration_in_seconds do
-      Faker::Number.between(from: 0, to: 5.minutes.seconds)
+      Faker::Number.between(from: 1, to: 5.minutes.seconds)
     end
     instrument
     user

spec/factories/section.rb

@@ -9,7 +9,7 @@
 
     end_time_hour { 0 }
     end_time_minute { 0 }
-    end_time_second { Faker::Number.between(from: 1, to: 59 ) }
+    end_time_second { Faker::Number.between(from: 2, to: [lesson.duration_in_seconds, 50].min ) }
 
     playback_speed { 0.5.step(by: 0.5, to: 2.0).to_a.sample }
   end