Bug 53468: Don't consume trailing IEW when skipping about siblings

getAboutSiblings in DOMUtils should skip over IEW between about-marked
elements in fosterable position. It did however also consume trailing IEW in
fosterable position, which led to the loss of newlines in round-tripping
between templated table cells as reported in bug 53468.

This is already covered by a parser test, but our selser testing currently
does not try to serialize each table cell individually.

TODO: Improve selser testing so that transclusions can be scheduled for
per-node serialization too. Alternatively, work around this limitation by
adding sibling cells that can be selser-modified.

Change-Id: I28e132d72b4c9927cdf85f239a9adae3374154ca
Merge "Bug 53468: Don't consume trailing IEW when skipping about siblings"