CONFIG_MULTITHREADING=n is not tested on hardware platforms #34840
Description
Describe the bug
Single thread support in Zephyr (CONFIG_MULTITHREADING=n) has been recently de-deprecated, however the configuration is not currently enabled in the tree for real hardware. There are several samples and tests that enable CONFIG_MULTITHREADING=n, however they only build and/or run in QEMU targets. If we claim support for the feature, we need to support it at least on a set of platforms and on a set of architectures, and to be testing this in CI.
Expected behavior
CONFIG_MULTITHREADING=n needs to be enabled for and tested on real hardware.