3.2 improve schema test coverage

[ralfhandl]

We switched to https://github.com/hyperjump-io/json-schema-coverage for schema test coverage, and the new tool is more diligent than the preliminary tool used so far, reporting branches that are not covered by the existing schema tests.

This PR brings schema test coverage back up to 100%.

It includes the changes in

• v3.2-dev: update from dev #4779

which can't be merged due to the insufficient schema test coverage.

It does not change the spec.

It does not change the schemas.

It only adds test cases.

• schema changes are included in this pull request
• schema changes are needed for this pull request but not done yet
• no schema changes are needed for this pull request

[handrews]

I'm concerned that the actual validation coverage is not as thorough as the files make it appear- I have commented on one such example. This all depends on how the JSON Schema implementation handles things, and whether we expect this suite to be usable with other implementations. If we want it to work with any conforming JSON Schema implementation, we need to only test one failure condition per case.

[ralfhandl]

@handrews all test cases are now documented

[karenetheridge]

Given the number of commits in this PR, including multiple merge commits, could this be squashed at merge time? There should be an option for it right in the merge button. TIA!

[ralfhandl]

Given the number of commits in this PR, including multiple merge commits, could this be squashed at merge time?

I've squashed my last commits after the merge-commit from v3.2-dev.

I'm a bit hesitant to squash more because that may cause problems with merging the overdue PR

• v3.2-dev: update from dev #4787

which contains six commits that are also in this PR.

[ralfhandl]

There should be an option for [squash merging] right in the merge button

After a discussion in the TDC on preserving the commit history of PRs we disabled squash merging and rebase merging for this repo.