Add dir-shadow-42 to Text Directionality focus area.

[dbaron]

In web-platform-tests/wpt#44621 (also see whatwg/html#10097 and whatwg/html#10140) I effectively split the dir-shadow-41.html test into two tests, creating a new dir-shadow-42.html test, to make sure that what it used to be testing was still tested appropriately. (The new test has the directions swapped relative to the old one, to better distinguish inheritance from an ltr default.)

I think it would be good to add the new test to the Text Directionality focus area.

[nairnandu]

@jgraham can you review for Gecko?
@nt1m can you review for WebKit?

[nt1m]

Seems fine: https://wpt.fyi/results/html/dom/elements/global-attributes?label=master&label=experimental&aligned&view=interop&q=dir-shadow-42.html

[dholbert]

@jgraham can you review for Gecko

I can review for Gecko; seems fine to me, based on @dbaron's description and @nt1m's link which seems to show everyone passing the new test.

[nairnandu]

@dbaron since there is agreement on this proposal, could you create a PR for the change?