Most browsers support the :nth-child selector by now, which allows us to
zebra-stripe the tables in CSS rather than making the python code
annotate every row with an 'a' or 'b' class.

While we're at it, stripe the pass/fail/crash results too.  Based on
some CSS fragments, this was the intent all along, but didn't work.

This also changes the striping slightly: it now stripes purely based on
whether the row is even/odd, rather than striping on a per-group basis.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Dylan Baker <baker.dylan.c@gmail.com>