t/unit-tests: implement clar specific reftable test helper functions

t/unit-tests: implement clar specific reftable test helper functions

Helper functions defined in `t/unit-tests/lib-reftable.{c,h}` are
required for the reftable-related test files to run. In the current
implementation these functions are designed to conform with our
homegrown unit-testing structure. So in other to convert the reftable
test files, there is need for a clar specific implementation of these
helper functions.

Implement equivalent helper functions in `lib-reftable-clar.{c,h}` to
use clar. These functions conform with the clar testing framework and
become available for all reftable-related test files implemented using
the clar testing framework, which requires them. This will be used by
subsequent commits.

Signed-off-by: Seyi Kuforiji <kuforiji98@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>