t-reftable-block: use reftable_record_key() instead of strbuf_addstr()

t-reftable-block: use reftable_record_key() instead of strbuf_addstr()

In the current testing setup, the record key required for many block
iterator functions is manually stored in a strbuf struct and then
passed to these functions. This is not ideal when there exists a
dedicated function to encode a record's key into a strbuf, namely
reftable_record_key(). Use this function instead of manual encoding.

Mentored-by: Patrick Steinhardt <redacted>
Mentored-by: Christian Couder <redacted>
Signed-off-by: Chandra Pratap <redacted>
Signed-off-by: Junio C Hamano <redacted>