Add test for unused class to shrinker-test

[Marcono1234]

Purpose

Extends tests of shrinker-test for unused classes

Description

Based on #2448

Adds a test case for a class with a field annotated with @SerializedName, but which is not actually used anywhere. The default Gson ProGuard rules should allow this class to be removed.

Note: This actually works as intended for R8, but not for ProGuard which keeps the class. Not sure if it is worth investigating that further at the moment.

Checklist

• New code follows the Google Java Style Guide
• If necessary, new public API validates arguments, for example rejects null
• New public API has Javadoc
  • Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• If necessary, new unit tests have been added
  • Assertions in unit tests use Truth, see existing tests
  • No JUnit 3 features are used (such as extending class TestCase)
  • If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• mvn clean verify javadoc:jar passes without errors

[Marcono1234]

@eamonnmcmanus, just to avoid any confusion, this does not address the open points I mentioned in #2448 regarding updating comments and adding a separate test for a class without no-args constructor.

This here adds a different test case, and I simply decided to base it on #2448 to reduce merge conflicts later.

[eamonnmcmanus]

Thanks, still looks good!