tests: kernel: Add qemu_leon3 to no-multithreading tests

Add qemu_leon3 target to the tests that list below. These set
CONFIG_MULTITHREADING=n.

- tests/kernel/mem_heap/mheap_api_concept
- tests/kernel/mem_slab/mslab_api
- tests/kernel/threads/no-multithreading
- tests/kernel/timer/timer_api

Signed-off-by: Martin Åberg <martin.aberg@gaisler.com>