Minor fixes for test_bitmapset.c

Minor fixes for test_bitmapset.c

1. Make it so test_random_operations() can accept a NULL to have the
   function select a random seed.
2. Widen the seed parameter of test_random_operations() to bigint.
   Without that, it'll be impossible to run the function with a seed
   which was selected by GetCurrentTimestamp(), and if a randomly
   selected seed ever results in a failure, we'll likely want to run
   with the same seed to debug the issue.
3. Report the seed in the error messages in test_random_operations().
   If the buildfarm were ever to fail there, we'd certainly want to know
   what this was.
4. Add CHECK_FOR_INTERRUPTS() to test_random_operations().  Someone might
   run with a large num_ops and they'd have no way to cancel the query.
5. Minor cosmetic fixes; header order and whitespace issue.

To allow #1, the STRICT modifier had to be removed.  The additional
prechecks were added as I didn't see how else to handle someone passing
those parameters as NULL.

Author: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Greg Burd <greg@burd.me>
Discussion: https://postgr.es/m/CAApHDvrDW9W72vAr7h7XeCu7+Qz-_Vff02Q+RPPuVeM0Qf0MCw@mail.gmail.com