tests: fix test_static_timing route removal

tests: fix test_static_timing route removal

On the first step, the test creates 10000 static routes. It passes 10000
to `get_ip_networks` and it generates 10000 /22 routes.

On the fourth step, the test tries to remove 5000 previously created
routes. It passes 5000 to `get_ip_networks` and here starts the problem.
Instead of generating 5000 /22 routes, it generates 5000 /21 routes. And
the whole step is a no-op, we constantly see the following logs:
```
% Refusing to remove a non-existent route
```

To consistently generate same routes, `get_ip_networks` must always use
the same prefix length.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>