tests: log_core_additional: Disable log_user test for Zephyr SDK

This commit disables the `logging.add.log_user` test when building with
the Zephyr SDK because it fails for all emulate-able platforms at
run-time.

Revert this commit when the GitHub issue #49213, which tracks this bug,
is fixed.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>