t/test_tree_copy.t error

[Gavin Smith]

t/test_tree_copy.t, which is run as part of the test suite, prints an
error:

$ perl -w t/test_tree_copy.t
1..7
ok 1 - modules loading
ERROR: copy_tree: no tree_document_descriptor
ok 2 # skip test perl not XS
ok 3 - tree to texi and original match
ok 4 - tree and copy to texi match
ok 5 - tree after sectioning and copy to texi match
ok 6 - manual tree and copy to texi match
ok 7 - manual tree and copy to texi match

However, the test suite still passes.