cmake: accommodate for `UNIT_TEST_SOURCES`

As part of 9bbc981c6f2 (t/unit-tests: finalize migration of
reftable-related tests, 2025-07-24), the explicit list of
`UNIT_TEST_PROGRAMS` was turned into a wildcard pattern-derived list.

Let's do the same in the CMake definition.

This fixes build errors with symptoms like this:

CMake Error at CMakeLists.txt:132 (string):
string sub-command REPLACE requires at least four arguments.
Call Stack (most recent call first):
CMakeLists.txt:1037 (parse_makefile_for_scripts)

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