Increase data directory test coverage

[vassudanagunta]

Created these tests as I was trying to solve a number of related issues (see below). A comprehensive fix is not trivial because Hugo uses three different three unmarshalers that behave differently for the same data structures. I will likely revert to fixing the regression first before making another attempt at the big picture. To keep changes incremental and easier to review, I'm pushing these test changes first, and I'm initially basing them on the v0.34 branch to demonstrate that the new tests all pass for that release.

@bep or @moorereason, please review this test code. You can merge it onto v0.34 if you wish, but not onto v0.35, as a couple of the new tests will fail there. Otherwise I'll include this commit along with a fix on a separate PR to master.

In this commit:

• Adds retro-coverage for regression: Fail to unmarshal JSON arrays #4361 (regression introduced in v.035)
• Verifies open issues parser: Fix YAML maps key type #4138, Allow other types than maps in .Site.Data #3890, Array data source in the path of another can result in golang Panic #4366, Collection key type in YAML breaks map range order #4083
• Removes test reliance on the very code it is testing (hugo/parser package).
  Expected results are now all built manually / are more precise.
  Tests can run against different versions (no linkage errs)

[vassudanagunta]

@bep Since this PR was just to demo the new tests against v0.34, and since #4373 includes these new tests plus a v0.35 fix, you can just close the PR if and when you merge that one.

[github-actions]

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