tests: net: coap_oscore: Select salt-only source via Kconfig

Drive the salt-only test source selection from CONFIG_TEST_OSCORE_SALT_ONLY
instead of a bare CMake cache variable so it reaches the application image
under sysbuild, where the previous -D only affected the top-level build and
left the /ns targets building main.c and failing to link
nvm_read/write_ssn.

Assisted-by: Cursor:claude-opus-4.8
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>