Add Descriptive Headings to Test Instructions and Criteria

[jkva]

This PR addresses the following item from PAC's "ARIA-AT App Screen Reader Accessibility Observations" document:

Within the content of a test, the lists of instructions, commands and assertions do not have accessible names, to help screen reader users understand their purpose and distinguish between the many lists on the page.

@jscholes this PR only covers the headings for test instructions and success criteria; the assertions table heading is covered by #375.

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Effective changes:

• add a h2 before "Test Instructions"
• describe the instructions ol using idref
• add a h2 before "Success Criteria"
• describe the criteria ol using idref

[jscholes]

@jkva I'm concerned about the heading hierarchy overlaps between this, #377, and #376. For instance, if #376 increases the level of what used to be a duplicate h1, it seems like the headings introduced in this PR (and maybe #377) should be higher? May be worth a double check.

[jkva]

@jscholes I've been aware of that as some of these PRs do indeed intersect. I'll merge a few together in a general "Heading Structure Improvements" PR.

[alflennik]

Just let us know when you think this is ready for our review @jkva.

#369 is no longer relevant (see its closing comment), hence this PR is still valid.

[jkva]

@alflennik given the overarching PR #396 was closed (since some other PRs it contained have since been merged), this PR should still be merged, hence please review.