Test C DR conformance (part one of many)

This starts to fill out the C DR status page with information
determined from tests. It also starts to add some test coverage for the
DRs we can add tests for (some are difficult as not all C DRs involve
questions about code and some DRs are about the behavior of linking
multiple TUs together).

Note: there is currently no automation for filling out the HTML page
from test coverage like there is for the C++ DRs, but this commit
attempts to use a similar comment style in case we want to add such a
script in the future.