t-reftable-merged: add test for REFTABLE_FORMAT_ERROR

t-reftable-merged: add test for REFTABLE_FORMAT_ERROR

When calling reftable_new_merged_table(), if the hash ID of the
passed reftable_table parameter doesn't match the passed hash_id
parameter, a REFTABLE_FORMAT_ERROR is thrown. This case is
currently left unexercised, so add a test for the same.

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