util: Add a unit test for the m5 util's "sum" command.

util: Add a unit test for the m5 util's "sum" command.

This change adds the plumbing for and then implements a unit test for
the "sum" command. Despite the fact that this command is very simple,
there are a few things to verify.

1. That args are passed in the right positions.
2. That the number of arguments is checked correctly.
3. That the output to std::cerr is correct.

Change-Id: I71cd473b78fb710cac94df2d70c8d6dc76e5a037
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27566
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>