t-reftable-readwrite: add test for known error

t-reftable-readwrite: add test for known error

When using reftable_writer_add_ref() to add a ref record to a
reftable writer, The update_index of the ref record must be within
the limits set by reftable_writer_set_limits(), or REFTABLE_API_ERROR
is returned. This scenario is currently left untested. Add a test
case 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>