[io grant] Change \0 roundtrip test to \t roundtrip test

NUL bytes aren't allowed in paths, so this test is in direct
contradiction with the security bug I'm trying to fix.

Since it's trying to test roundtrippage of whitespace, change it
to use use \t instead of \0.

https://irclog.perlgeek.de/perl6-dev/2017-04-10#i_14405645

S32-io/io-path.t

@@ -52,8 +52,8 @@ isa-ok $path.IO,   IO::Path, 'IO::Path.IO returns IO::Path';
 {
     my $perl = "/foo|\\bar".IO.perl;
     is $perl.EVAL.perl, $perl, "does $perl roundtrip?";
-    my $nul = "/foo\0bar".IO.perl;
-    is $nul.EVAL.perl, $nul, "does $nul roundtrip?";
+    my $tab = "/foo\tbar".IO.perl;
+    is $tab.EVAL.perl, $tab, "does $tab roundtrip?";
 }
 
 # RT #127989