Rewrite nsec3 system test to pytest (3/4)

Rewrite nsec3 system test to pytest (3/4)

This converts two test cases:

1. A zone that previously failed to load is now fixed. Make sure the
   zone is signed correctly with the right NSEC3 parameters.

2. Test case to ensure the salt is the same after a restart, i.e. no
   re-salting takes place. Previously we only tested with salt length
   0, this commit adds a test case for salt length 8 as well.