Tests: Refactor compareMetadata to handle both success and failure cases

[kfranqueiro]

This is a follow-up to #2065 to address #2065 (comment).

Changes

• Moves metadata failure case definition into samples.js where success cases are defined
  • Failure cases are distinguished by the presence of an errors array
• Refactors compareMetadata to be driven by samples.js rather than using a hard-coded (and long-winded) set of checks
  • This required removing properties from samples.js which were never being compared before, and correcting one test fixture which was supposed to be compared but never was until this refactor
  • I was also able to remove the try/catch by instead using nextTick - previously, it seemed that a try/catch was sometimes but not always necessary to prevent the process from hanging, seemingly dependent on whether the final check resolved synchronously
  • I also took the opportunity to remove an unused argument (only used by code that has been commented out for probably a long time), and a util function that can be replaced by deep.equal.

I have verified that assertion failures behave as expected by altering various lines in samples.js.

[deniak]

Nice simplification!