t/unit-tests: implement clar specific oid helper functions

t/unit-tests: implement clar specific oid helper functions

`get_oid_arbitrary_hex()` and `init_hash_algo()` are both required for
oid-related tests to run without errors. In the current implementation,
both functions are defined and declared in the
`t/unit-tests/lib-oid.{c,h}` which is utilized by oid-related tests in
the homegrown unit tests structure.

Adapt functions in lib-oid.{c,h} to use clar. Both these functions
become available for oid-related test files implemented using the clar
testing framework, which requires them. This will be used by subsequent
commits.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Seyi Kuforiji <kuforiji98@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>