Document (and split off) tests that require `>C89`

[edsko]

As of #1798 the default C standard for all tests is C89 (#1516); tests that require a later version are explicitly marked as such. We should document each of these cases; why do they require a specific C standard version. If it turns out that it's just one tiny part of the test that requires this, we should consider splitting off that part of the test into its own header, and then "reset" the required C standard for the "parent" test.

[edsko]

Marking this as a low priority enhancement for now; we can bump if and when this becomes more of an issue.