cmake: generalize the handling of the UNIT_TEST_OBJS list

In a15d446 (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 8727215 (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 e7e6d74 (Makefile: stop listing test library objects twice,
2024-09-09), 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 4d76732 (t/unit-tests: introduce reftable library,
2024-09-09), 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>

contrib/buildsystems/CMakeLists.txt

@@ -975,13 +975,14 @@ parse_makefile_for_sources(test-reftable_SOURCES "REFTABLE_TEST_OBJS")
 list(TRANSFORM test-reftable_SOURCES PREPEND "${CMAKE_SOURCE_DIR}/")
 
 #unit-tests
-add_library(unit-test-lib OBJECT ${CMAKE_SOURCE_DIR}/t/unit-tests/test-lib.c)
-add_library(unit-test-lib-oid OBJECT ${CMAKE_SOURCE_DIR}/t/unit-tests/lib-oid.c)
+parse_makefile_for_sources(unit-test_SOURCES "UNIT_TEST_OBJS")
+list(TRANSFORM unit-test_SOURCES REPLACE "\\$\\(UNIT_TEST_DIR\\)/" "${CMAKE_SOURCE_DIR}/t/unit-tests/")
+add_library(unit-test-lib STATIC ${unit-test_SOURCES})
 
 parse_makefile_for_scripts(unit_test_PROGRAMS "UNIT_TEST_PROGRAMS" "")
 foreach(unit_test ${unit_test_PROGRAMS})
 	add_executable("${unit_test}" "${CMAKE_SOURCE_DIR}/t/unit-tests/${unit_test}.c")
-	target_link_libraries("${unit_test}" unit-test-lib unit-test-lib-oid common-main)
+	target_link_libraries("${unit_test}" unit-test-lib common-main)
 	set_target_properties("${unit_test}"
 		PROPERTIES RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/t/unit-tests/bin)
 	if(MSVC)