Enable writable tests for sharded databases
Writing to a sharded WritableDatabase has been supported since 1.3.2,
but the test harness wasn't running many of the tests that could be
with a sharded WritableDatabase - now those requiring "writable" are
run, which uncovered a bug in replace_document(term, doc) on a
sharded database (fixed by this commit) and also a bug with
get_termfreq() on a TermIterator from a Document, which currently
returns the termfreq for just the shard the doc is in (not yet fixed -
adddoc2 and adddoc5 trigger this but aren't run under multi for
now).