Improve ABORT handling in RTEMS unit test

[jphickey]

Is your feature request related to a problem? Please describe.
If an unit testing error occurs that causes the test to abort, the implementation just calls the system abort() function. In RTEMS, this causes the entire system/kernel to shutdown. This leaves no ability to use the shell to check the system state or diagnose what went wrong.

Describe the solution you'd like
Defer handling of the abort to the BSP. In RTEMS, when the shell is available, this should just suspend the calling task to stop the tests, allowing the shell to continue being used.

Additional context
This used to be the case, but probably an artifact of merging the UT BSP with regular BSP that this got lost in translation, and it only shows up if an abort failure occurs. Fairly trivial/easy to add it back in though.

Requester Info
Joseph Hickey, Vantage Systems, Inc.