Improves coverage stats

[hjJunior]

Task

• Adds tests for SwiperPluginConfig
• Adds tests for SwiperPluginView
• Adds tests for CustomLayoutOption

I also added @required which helps with code completion on IDEA

Before

Overall coverage rate:
  lines......: 59.7% (397 of 665 lines)
  functions..: no data found

After

Overall coverage rate:
  lines......: 62.3% (420 of 674 lines)
  functions..: no data found

Why I added two new dependencies?

Mockito

Mockito is the most popular package on Dart which allows to mock classes dependencies and also enables some matches on tests such if the function has been called

Equatable

Since we can't compare direct two objects without overriding equals method on each class, this is a fast way to "override" it, this is required to enable flutter test to compare on expect tests