Change test output file from out.pdf to a.pdf for cleanup

The cleanup() method automatically removes certain files, but not
out.pdf. Using a.pdf prevents the file from being left behind after
the tests run.

qpdf/qtest/page-errors.test

@@ -21,18 +21,18 @@ $td->runtest("handle page no with contents",
              {$td->FILE => "page-no-content.out", $td->EXIT_STATUS => 3},
              $td->NORMALIZE_NEWLINES);
 $td->runtest("handle page with missing MediaBox",
-             {$td->COMMAND => "qpdf --static-id --empty --pages page-no-content.pdf -- out.pdf"},
+             {$td->COMMAND => "qpdf --static-id --empty --pages page-no-content.pdf -- a.pdf"},
              {$td->FILE => "page-missing-mediabox.out", $td->EXIT_STATUS => 3},
              $td->NORMALIZE_NEWLINES);
 $td->runtest("check output",
-             {$td->FILE => "out.pdf"},
+             {$td->FILE => "a.pdf"},
              {$td->FILE => "page-missing-mediabox-out.pdf"});
 $td->runtest("handle page with inherited MediaBox",
-             {$td->COMMAND => "qpdf --static-id --empty --pages page-inherit-mediabox.pdf -- out.pdf"},
+             {$td->COMMAND => "qpdf --static-id --empty --pages page-inherit-mediabox.pdf -- a.pdf"},
              {$td->STRING => "", $td->EXIT_STATUS => 0},
              $td->NORMALIZE_NEWLINES);
 $td->runtest("check output",
-             {$td->FILE => "out.pdf"},
+             {$td->FILE => "a.pdf"},
              {$td->FILE => "page-inherit-mediabox-out.pdf"});
 $td->runtest("check no type key for page nodes",
              {$td->COMMAND => "qpdf --check no-pages-types.pdf"},