Speed up test_ca_warn_report:test_objects

The same goals can be acheived by just testing the first page, instead
of the entire PDF, trimming approximately 20% off the total test suite's
execution time, although YMMV.

tests/test_ca_warn_report.py

@@ -31,9 +31,10 @@ def test_page_limiting(self):
             assert pdf.pages[1].page_number == 3
 
     def test_objects(self):
-        assert len(self.pdf.chars)
-        assert len(self.pdf.rects)
-        assert len(self.pdf.images)
+        p = self.pdf.pages[0]
+        assert len(p.chars)
+        assert len(p.rects)
+        assert len(p.images)
 
     def test_parse(self):