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>

util/m5/src/SConscript.native

@@ -25,6 +25,14 @@
 
 Import('*')
 
+env.Append(CPPPATH=Dir('.'))
+
 env.GTest('args')
 env.GTest('call_type')
 env.GTest('command', 'command.test.cc', 'command.cc', 'args.cc')
+
+command_tests = env.SConscript('command/SConscript.native', exports='env')
+
+for test in command_tests:
+    env.GTest('commands/%s' % test, 'command/%s.test.cc' % test,
+              'command/%s.cc' % test, 'args.cc', 'command.cc')