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

t-reftable-record: add ref 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 all the four record types (ref, log, obj, index).

Add tests for this function in the case of ref records.

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>