Add tests to make sure that file names roundtrip correctly when they should

[AlexDaniel]

See this discussion: https://irclog.perlgeek.de/perl6-dev/2017-01-14#i_13921183

[jnthn]

It was fixed during the recent utf8-c8 rewrite in MoarVM. UTF8-C8 will now only decode stuff as UTF-8 if it's both valid UTF-8 and in NFC. Otherwise, it spits out synthetics to record the original bytes and allow them to be reconstructed. It's worth adding tests for this on filenames, but S32-str/utf8-c8.t also got some new tests covering this.