[v3] Updated migration documentation, added trait for testing migratrations

[kynx]

This supersedes #87 fleshing out the migration documentation to include:

• examples of adding normalised FQCN entries for factories
• clear up the changes required for $shareByDefault / $sharedByDefault
• provide example of handling plugin manager validation exceptions

It includes a trait that can be used when testing migrations to ensure aliases resolve, etc.

Note that tests will fail until #92 is merged

[weierophinney]

This should be in the src/Test/ directory, and have the namespace Zend\ServiceManager\Test. Why? Because Composer typically tries to install distribution builds, and our .gitattributes file sets up distribution builds to omit the test/ directory. As such, the trait will not be present for most users unless then install with the --prefer-source flag.

Move the trait, and update the docs, and I can merge. 👍

[kynx]

Thanks - I've moved the test trait as requested. I'll do a separate PR against release-2.7 so any tests using it run in both.

[weierophinney]

Cherry-picked to master for release with 3.0.3.