[v6.d REVIEW] Fix up fh-close-on-exit tests

The first write to a handle is unbuffered, so the original test
likely doesn't cover the bug.

Orig: 08ac054de

S32-io/io-handle.t

@@ -215,10 +215,13 @@ given make-temp-file() {
 subtest 'opened filehandles get closed on exit automatically' => {
     plan 2;
     my $path = make-temp-file;
-    is_run $path.perl ~ ｢.open(:w, :5000out-buffer).print: 'pass'; print 'pass'｣,
-        {:out<pass>, :err(''), :0status}, 'written into a file without closing';
+    is_run ｢my $fh := ｣ ~ $path.perl  ~ ｢.open: :w, :5000out-buffer;
+      $fh.print: 'pass'; $fh.print: '-pass2';
+      print 'pass'
+    ｣, {:out<pass>, :err(''), :0status}, 'written into a file without closing';
 
-    is-deeply $path.slurp, 'pass', 'file has all the content we wrote into it';
+    is-deeply $path.slurp, 'pass-pass2',
+        'file has all the content we wrote into it';
 }
 
 { # RT #131858