t/python: add extra OutputFormat coverage

Problem: The current OutputFormat python tests always assume output
headers are passed in.

Add some tests that do not pass output headers in.

Author: chu11

t/python/t0024-util.py

@@ -218,6 +218,11 @@ def test_filter(self):
         ).filter(items)
         self.assertEqual(fmt, "{i:>8} {f:5.2f}")
 
+        fmt = OutputFormat(
+            "?+:{i:>7} ?:{s:>6} ?+:{f:.2f}", headings=None
+        ).filter(items)
+        self.assertEqual(fmt, "{i:>8} {f:3.2f}")
+
     def test_sort(self):
         a = Item("a", 0, 2.2)
         d = Item("d", 33, 1.1)
@@ -270,6 +275,11 @@ def test_issue6530(self):
         ).filter(items)
         self.assertEqual(fmt, "{s:16.16} {i:7d} {f:5.2f}")
 
+        fmt = OutputFormat(
+            "+:{s:5.5} +:{i:4d} +:{f:.2f}", headings=None
+        ).filter(items)
+        self.assertEqual(fmt, "{s:16.16} {i:4d} {f:3.2f}")
+
     def test_copy(self):
         original = "+:{s:5.5} {i:4d} {f:.2f}"