[merge] fix up bad calls to build_tree

[Martin Pool]

Some recently-added tests were passing strings to
TestCase.build_tree() rather than lists of names.  This not only
creates the wrong files (by Python treating the string as a sequence
of characters), but also makes it fail when run without -O.  This
patch corrects the calls, and also changes the assertion to a manual
if/raise so it'll be more consistent.

-- 
Martin
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