Here is some useful information regarding the mpt_tests
With 500 keys, it is sufficient to use the parameters max_depth = 6 and max_key_byte_len = 3.
loose - in these tests, we use max_depth = 6 and max_key_byte_len = 32, key_byte_len = Some(key.len())
tight - in these tests, we use max_depth = proof.len() + slot_is_empty, max_key_byte_len = key.len(), key_byte_len = Some(key.len())
fixed - in these tests, we use max_depth = 6, max_key_byte_len = key.len(), key_byte_len = None