[test]: Modify test_sst_build_all() to add checks for first_key and last_key of SST

[Gleiphir2769]

Hi, when I implemented W1D5, I found that my code can not pass test_task2_storage_scan(). It is caused by wrong result returned by sst.first_key() and sst.last_key() because of my missing in SsTableBuilder.build()

I think it's better to add checks within test_sst_build_all() in week1_day4 so that we can ensure the correctness of the SST implementation. What do you think?