LayoutTest: fix inconsistent use of test function
LayoutTest: fix inconsistent use of test function

A test case added in 2017 used BDD instead of TDD style naming, which
worked in older versions of mocha but doesn't now.  Replace with
consistent naming.

Signed-off-by: Peter A. Bigot <650538893330be855f76604594b4820eb226769e@pabigot.com>

pabigot/buffer-layout,pabigot/buffer-layout