Avoid platform-dependent output from Data::Dumper.

Per buildfarm, the output from Data::Dumper for an IEEE infinity
is platform-dependent (e.g. "inf" vs "Inf").  Just skip that one
test case in the plperlu test; testing it on the plperl side is
coverage enough.  Fixes issue in commit 1731e3741.

contrib/jsonb_plperl/expected/jsonb_plperlu.out

@@ -78,10 +78,8 @@ INFO:  $VAR1 = '1';
  1
 (1 row)
 
-SELECT roundtrip('1E+131071');
-INFO:  $VAR1 = 'inf';
-ERROR:  cannot convert infinity to jsonb
-CONTEXT:  PL/Perl function "roundtrip"
+-- skip because Data::Dumper produces a platform-dependent spelling of infinity
+-- SELECT roundtrip('1E+131071');
 SELECT roundtrip('-1');
 INFO:  $VAR1 = '-1';
  roundtrip