TST: rename checknull_old -> test_checknull_old (#33546)

Author: simonjayhawkins

pandas/tests/dtypes/test_missing.py

@@ -564,7 +564,7 @@ def test_checknull(self):
         for value in never_na_vals:
             assert not libmissing.checknull(value)
 
-    def checknull_old(self):
+    def test_checknull_old(self):
         for value in na_vals:
             assert libmissing.checknull_old(value)