Data-Dumper: fix sortkeys.t test count for non-XS testing

This is a follow-up to 67201d9, which
reduced the number of tests without adjusting the skip count.

Previously, testing without XS available resulted in:

    t/sortkeys.t .................. Dubious, test returned 255 (wstat 65280, 0xff00)
    All 22 subtests passed
            (less 13 skipped subtests: 9 okay)
    ...
    /sortkeys.t                (Wstat: 65280 (exited 255) Tests: 24 Failed: 2)
      Failed tests:  23-24
      Non-zero exit status: 255
      Parse errors: Bad plan.  You planned 22 tests but ran 24.

Author: mauke

dist/Data-Dumper/t/sortkeys.t

@@ -11,7 +11,7 @@ use Testing qw( _dumptostr );
 
 run_tests_for_sortkeys();
 SKIP: {
-    skip "XS version was unavailable, so we already ran with pure Perl", 13 
+    skip "XS version was unavailable, so we already ran with pure Perl", 11
         if $Data::Dumper::Useperl;
     local $Data::Dumper::Useperl = 1;
     run_tests_for_sortkeys();