[v6.d REVIEW] Expand IO::Spec::Unix.basename

Include a test with backslash, which isn't a path separator on *nix

Author: zoffixznet

S32-io/io-spec-unix.t

@@ -197,7 +197,7 @@ else {
 
 subtest '.basename' => {
     my @tests = '' => '', '.' => '.', '/' => '', 'foo/' => '', '/.' => '.',
-        'foo/.' => '.', 'foo/..' => '..', 'foo/...' => '...',
+        'foo/.' => '.', 'foo/..' => '..', 'foo/...' => '...', ｢y/\z｣ => ｢\z｣,
         'bar/♥foo' => '♥foo', '♥foo' => '♥foo', '♥foo/..' => '..';
 
     plan +@tests;