Add mockito-inline dependency

[mpalourdio]

Hello,

This PR adds mockito-inline to dependencies. Mockito-inline is a convenient module that allows final classes mocking.

As Spring Boot 2.x now has mockito>2 by default, I think this could be a welcome addition.

[wilkinsona]

I wonder if we should add mockito-inline to spring-boot-starter-test?

[mpalourdio]

I can submit a PR to spring-boot-starter-test if ok. Just let me know.

[philwebb]

Given this note from their WIKI, I'd be tempted to not add this by default (at least not yet)

Mocking of final classes and methods is an incubating, opt-in feature. It uses a combination of Java agent instrumentation and subclassing in order to enable mockability of these types. As this works differently to our current mechanism and this one has different limitations and as we want to gather experience and user feedback, this feature had to be explicitly activated to be available

[snicoll]

There is an experimental module in JUnit5 that we haven't added at all in our dependency management so if we want to be consistent we should either reject this or add the one of Junit5

[philwebb]

I'm personally happy for it to be in the dependency management, I just don't think we should include it in our starter.

[wilkinsona]

That sounds like the right balance to strike to me.