t/unit-tests: convert reftable stack test to use clar

t/unit-tests: convert reftable stack test to use clar

Adapt reftable stack test file to use clar by using clar assertions
where necessary.

This marks the end of all unit tests migrated away from the
`unit-tests/t-*.c` pattern, there are no longer any files matching that
glob. Remove the sanity check for `t-*.c` files to prevent Meson
configuration errors during CI and local builds.

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