Handle failure indications output by zdump 2022a.

zdump in 2022a outputs 'gmtime failed' or 'localtime failed' with the
timestamp in question when the local or UT cannot be determined for the
timestamp. Previous versions output 'NULL'.

test/tc_definitions.rb

@@ -178,7 +178,7 @@ class TCDefinitions < Minitest::Test
         end
       end
     else
-      assert_match(/\s=\sNULL\z/, line, "Unexpected zdump format")
+      assert_match(/(?:-?\d+\s\(gmtime failed\)\s=\s|\s=\s(?:NULL|-?\d+\s\(localtime failed\))\z)/, line, 'Unexpected zdump format')
     end
   end