tests: Add test for the m5-exit instruction.

Change-Id: I92a589b267ce659b6fbcf710043436b84fcb1c63
Signed-off-by: Sean Wilson <spwilson2@wisc.edu>
Reviewed-on: https://gem5-review.googlesource.com/4423
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>