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>