[tests] Print status explicitly -

Some simulators do return the exit code, but Qemu doesn't.
This will make it easier to grep the result.
[build] Test suite name should not need extension
[tests] Rename `coreunit.c` to simply `core.c`
[build] This makefile should only be included once
[build][tests] Compile for Cortext-M3, run on Qemu