Slowly start doing more HTML output tests, in this case for the

Slowly start doing more HTML output tests, in this case for the
interaction of .nf and .RS, related to man_macro.c rev. 1.106.

HTML regression testing is tricky because it is extremely prone to
over-testing, i.e. unintentional testing for volatile formatting
details which are irrelevant for deciding whether the HTML output
is good or bad.  Minor changes to the formatter - which is still
heavily under development - might result in the necessity to
repeatedly adjust many test cases.

Then again, HTML syntax rules are so complicated that without
regression testing, the risk is simply too high that later changes
will re-introduce issues that were already fixed earlier.  Let's
just try to design the tests very carefully in such a way that
the *.out_html files contain nothing that is likely to change, and
defer testing in cases where the HTML output is not yet clean enough
to allow designing tests in such a way.