Fix W1D6 test

[yyin-dev]

The assert is changed to: assert!(min_num <= iter.num_active_iterators() && iter.num_active_iterators() < max_num);

Reasoning:
The sstables should look like the following (format: (id, first_key, last_key)):

[(0, b"00001", b"00999"), 
(1, b"01000", b"01999"), 
(2, b"02000", b"02999"), 
(3, b"03000", b"03999"), 
(4, b"04000", b"04999"), 
(5, b"05000", b"05999"), 
(6, b"06000", b"06999"), 
(7, b"07000", b"07999"), 
(8, b"08000", b"08999"), 
(9, b"09000", b"09999")]

Given query

let iter = storage
        .scan(
            Bound::Included(format!("{:05}", 5000).as_bytes()),
            Bound::Excluded(format!("{:05}", 6000).as_bytes()),
        )
        .unwrap();

it's correct for the filter to only return the Sstable with id=5, and thus min_num <= iter.num_active_iterators().

[skyzh]

Thanks!