Fix incorrect object used

The IO::Path::Unix is already tested in the S32-io/io-path-unix.t [^1],
and this appears to be a copy-paste error, as we need to
test ::Cygwin object in this test.

[1] https://github.com/perl6/roast/blob/7428736eeb86a40e5f1cef594163ff50e061253f/S32-io/io-path-unix.t#L63-L68

S32-io/io-path-cygwin.t

@@ -76,12 +76,12 @@ is IO::Path::Cygwin.new("\\").parent,            "/",            "parent of root
 is IO::Path::Cygwin.new("/").child('foo'),    "/foo",        "append to root";
 is IO::Path::Cygwin.new(".").child('foo'),    "foo",        "append to cwd";
 
-my $numfile = IO::Path::Unix.new("foo/file01.txt");
+my $numfile = IO::Path::Cygwin.new("foo/file01.txt");
 is $numfile.succ,    "foo/file02.txt", "succ basic";
 is $numfile.succ.succ,    "foo/file03.txt", "succ x 2";
 is $numfile.pred,    "foo/file00.txt", "pred basic";
-is IO::Path::Unix.new("foo/()").succ, "foo/()", "succ only effects basename";
-is IO::Path::Unix.new("foo/()").succ, "foo/()", "pred only effects basename";
+is IO::Path::Cygwin.new("foo/()").succ, "foo/()", "succ only effects basename";
+is IO::Path::Cygwin.new("foo/()").succ, "foo/()", "pred only effects basename";
 
 # RT#128840
 isnt IO::Path::Cygwin.new('-a').absolute, '',