Unify the many copies of diff_with_file in one source
file, and patch it to detect diff at runtime and fall
back to a simple strcmp if we don't have it. Make all
tests use this new testutils.c, and stop requiring
diff for building the tests.
This should let us allow to build on Windows with the
default value for -Dbuild-tests.
CSS nodes have a linear sibling relationship; this is supposed
to correspond to left-to-right placement in horizontal arrangements.
This commit explicitly sets the text direction to rtl if the
filename ends in .rtl.ui, so we can test differences in node
tree layout between text directions.
