Use Guava testlib for collection tests

[Marcono1234]

Purpose

Use the Guava collection testlib for Gson's own collection classes

Description

Use the Guava collection testlib for Gson's own collection classes NonNullElementWrapperList and LinkedTreeMap, covered through the tests for JsonArray#asList and JsonObject#asMap, as well as a test for LinkedTreeMap.

I don't have that much experience with the Guava testlib yet, so any feedback or recommendations are appreciated!

Checklist

• New code follows the Google Java Style Guide
  This is automatically checked by mvn verify, but can also be checked on its own using mvn spotless:check.
  Style violations can be fixed using mvn spotless:apply; this can be done in a separate commit to verify that it did not cause undesired changes.
• 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

[cpovirk]

Thanks also for the Guava bug report, which I might not look at in detail for a while but which sounds concerning.

[cpovirk]

We typically use the GENERAL_PURPOSE features when applicable. There's such a constant for List and also for Map. I haven't confirmed whether it applies here, and it's fine to use an explicit list instead even if it would apply, but I wanted to at least point it out.

[Marcono1234]

Thanks for the hint! I wasn't completely sure whether they were suitable because CollectionFeature.GENERAL_PURPOSE says:

Features supported by general-purpose collections - everything but RESTRICTS_ELEMENTS.

See java.util.Collection the definition of general-purpose collections.

And the List and Map here are not 'general-purpose' in the sense of the java.util.Collection definition.