Avoid a panic when handling collapsing borders (fixes #7144)

There is no need to compute next_collapsed_borders_in_block_direction for all
kind of flows when it's used only in the table row case. That also avoids a panic
when the next child is a table colgroup (this should not happen when iterating
over a table row).

tests/wpt/metadata-css/css21_dev/html4/table-backgrounds-bc-cell-001.htm.ini

@@ -1,3 +1,3 @@
 [table-backgrounds-bc-cell-001.htm]
   type: reftest
-  expected: TIMEOUT
+  expected: FAIL