t/op/sub.t: silence new warning

A 7-year old test has recently started emitting this warning:

    Lexical subroutine &b masks previously declared package subroutine

because a new warning has been added to core.

It's not 100% clear to me whether the test itself was *relying* on a
package subroutine having the same name or not, but given that it was a
test which crashed rather than had well-defined "bad" behaviour", I
decided it was best to leave the test exactly as-is, and just silence
the warning.

t/op/sub.t

@@ -423,6 +423,8 @@ eval '
    {my ($a,$b,$c,$d,$e,$f,$g,$h,$i,$j,$k,$l,$m,$n,$o,$p,$q,$r,$s,$t,$u);}
    {my ($a,$b,$c,$d,$e,$f,$g,$h,$i,$j,$k,$l,$m,$n,$o,$p,$q,$r,$s,$t,$u);}
    {my ($a,$b,$c,$d,$e,$f,$g,$h,$i,$j,$k,$l,$m,$n,$o,$p,$q,$r,$s,$t,$u);}
+   # avoid "Lexical subroutine &b masks previously declared package subroutine"
+   no warnings "shadow";
    CORE::state sub b; sub d { sub b {} sub d }
  ';
 eval '()=%e; sub e { sub e; eval q|$x| } e;';