Tests should throw a meaningful error for objects that don't exist

[jnasbyupgrade]

At least when it comes to functions that test function metadata, most all of them suffer from this:

+# Failed test 3: "Function _task.task__run_for(text, int, task.task_options, boolean) should be security definer"
+#     (test result was NULL)

(test result was NULL) is rather confusing. If you try to run a test on an object that doesn't exist, you should get either a meaningful failure (ie, RAISE), or at least the test output should be a descriptive failure (ie, the equivalent of fail( 'function ... does not exist' ).