cmake: generalize the handling of the `UNIT_TEST_OBJS` list

cmake: generalize the handling of the `UNIT_TEST_OBJS` list

In a15d4465a991 (cmake: also build unit tests, 2023-09-25), I
accommodated the CMake definition. Seeing that a `UNIT_TEST_OBJS` list
was introduced that was built by transforming the `UNIT_TEST_PROGRAMS`
list and then adding a single, hard-coded file
("t/unit-tests/test-lib.c"), I decided to hard-code that in the CMake
definition, too.

The reason why I hard-coded it instead of imitating the
`parse_makefile_for_sources()` paradigm that was used elsewhere when
using the `Makefile` as source of truth for given lists of files: This
function expects _only_ hard-coded values, and that transformed
`UNIT_TEST_PROGRAMS` list complicated everything.

In 872721538c26 (cmake: fix build of `t-oidtree`, 2024-07-12), I
accommodated the CMake definition again, after seeing that the
`UNIT_TEST_OBJS` was still defined via that transformed list but now
appending _two_ hard-coded files ("t/unit-tests/lib-oid.c" joined the
fray).

In 428672a3b16 (Makefile: stop listing test library objects twice,
2024-09-16), the `Makefile` was changed so that `UNIT_TEST_OBJS` is
finally only constructed using hard-coded file names just like the other
`*_OBJS` variables. I missed that and therefore did not adjust the CMake
definition. Besides, the code was working, so there was no real need to
adjust it.

With a4f50bb1e9b (t/unit-tests: introduce reftable library, 2024-09-16),
however, the `UNIT_TEST_OBJS` list became a trio, and the CMake
definition has to be adjusted again. Now that we can use the
`parse_makefile_for_sources()` function without many complications,
let's do that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>