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 <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Chandra Pratap

t/unit-tests/t-reftable-readwrite.c

@@ -774,6 +774,11 @@ static void t_write_key_order(void)
 	check(!err);
 	err = reftable_writer_add_ref(w, &refs[1]);
 	check_int(err, ==, REFTABLE_API_ERROR);
+
+	refs[0].update_index = 2;
+	err = reftable_writer_add_ref(w, &refs[0]);
+	check_int(err, ==, REFTABLE_API_ERROR);
+
 	reftable_writer_close(w);
 	reftable_writer_free(w);
 	strbuf_release(&buf);