Update the status::renames test to create an NFD format filename in the
core.precomposedunicode tests.

Previously, we would create an NFC format filename.  This was to take
advantage of HFS+ filesystems, which always use canonically decomposed
formats, and would actually write the filename to disk as an NFD
filename.  So previously, we could create an NFC filename, but read it
normally as an NFD filename.

But APFS formats do not force canonically decomposed formats for
filenames, so creating an NFC filename does not get converted to NFD.
Instead, the filename will be written in NFC format.  Our test,
therefore, does not work - when we write an NFC filename, it will
_remain_ NFC.

Update the test to write NFD always.  This will ensure that the file
will actually be canonically decomposed on all platforms:  HFS+, which
forces NFD, and APFS, which does not.

Thus, our test will continue to ensure that an NFD filename is
canonically precomposed on all filesystems.