Add a (broken) Acid2 reftest. Fix #1993

The rendering is incorrect
and the test is marked as flaky on Linux or on GPU rendering,
but this should help catch further regressions.

The test is modified to add the following CSS:

    .intro { display: none }
    html #top { margin-top: 0 }

To disable the "scrolling" part of the test.

The reference is modified to match the current rendering of the test:

* Rows 4 and 5 (eyes) have a red background.
* The nose is not quite where it should be.
* Row 12’s height is 3px more than it should be.

(See http://www.webstandards.org/action/acid2/guide/ for row numbers.)

The nose issue seems be related to rounding vs truncating
when pixel-snapping borders, but this is only a guess.

src/test/ref/basic.list

@@ -94,3 +94,11 @@ flaky_cpu == linebreak_simple_a.html linebreak_simple_b.html
 == block_replaced_content_a.html block_replaced_content_ref.html
 == block_replaced_content_b.html block_replaced_content_ref.html
 == root_margin_collapse_a.html root_margin_collapse_b.html
+
+# Should be == with expected failure:
+!= ../html/acid2.html#top acid2_ref.html
+
+# Should be != with expected failure:
+# FIXME: use the real test when pixel-snapping for scrolling is fixed.
+#== ../html/acid2.html#top acid2_ref_broken.html
+flaky_gpu,flaky_linux == acid2_noscroll.html acid2_ref_broken.html