test: Add JsonWriter test from GSON repo

[fractalwrench]

Goal

Improve code coverage for JsonWriter by using existing tests in GSON repository. This should give more confidence to update the library in future.

Changeset

Added

A test from the GSON repo.

Discussion

Alternative Approaches

I've done a little monkey-patching by adding Test annotations to get this running on our current setup. I have also edited testTopLevelValueTypes as the API syntax appears to have changed in later versions of GSON, and omitted testNonFiniteDoublesWhenLenient entirely as it appears our notifier contains an older version of JsonWriter which is affected by this bug.

Outstanding Questions

Should we also update JsonWriter with this change?

Review

For the submitter, initial self-review:

• Commented on code changes inline explain the reasoning behind the approach
• Reviewed the test cases added for completeness and possible points for discussion
• A changelog entry was added for the goal of this pull request
• Check the scope of the changeset - is everything in the diff required for the pull request?
• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

For the pull request reviewer(s), this changeset has been reviewed for:

• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language

[coveralls]

Coverage increased (+0.6%) to 77.212% when pulling f742fe5 on add-jsonwriter-test into 4e1727a on master.