Fix JSON array-based data file handling regression

[vassudanagunta]

Fixes #4361, a regression introduced in Hugo v0.35.

New test code (first commit)

The first commit includes new test code that was also submitted via #4372 against the v0.34 branch to demonstrate that the updates tests did in fact all pass for that release. They will fail in this PR, against v0.35 code, until the second commit with the fix is added.

The new tests:

• Add retro-coverage for regression: Fail to unmarshal JSON arrays #4361, the regression fixed by the PR
• Verify 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/Convert YAML to JSON with menu fails #2441
• Remove 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)

regression fix (second commit)

A comprehensive fix is not trivial because Hugo uses three different three unmarshalers that behave differently for the same data structures. In the interests of fixing the regression ASAP, this PR will only address the regression.

[github-actions]

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