t-reftable-record: add obj tests for reftable_record_is_deletion()

t-reftable-record: add obj tests for reftable_record_is_deletion()

reftable_record_is_deletion() is a function defined in
reftable/record.{c, h} that determines whether a record is of
type deletion or not. In the current testing setup, this function
is left untested for two of the four record types (obj, index).

Add tests for this function in the case of obj records.
Note that since obj records cannot be of type deletion, this function
must always return '0' when called on an obj record.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Acked-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>