kselftest: timers: Fix const correctness

Make timespec pointers, pointers to const in checklist function. As a
consequence, make list parameter in checklist function pointer to const
as well. Const-correctness increases readability.

Improvement was found by running cppcheck tool on the patched file as
follows:
```
cppcheck --enable=all \
	tools/testing/selftests/timers/threadtest.c \
	--suppress=missingIncludeSystem \
	--suppress=unusedFunction
```

Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Piotr Zalewski <pZ010001011111@proton.me>
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>