util: Add a "command" unit test for the m5 utility.

This tests the common "command" machinery, but not the individual
commands themselves.

Change-Id: I92769b4cef8210458786e60fd3c01e8e787fb9b2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27563
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>

util/m5/src/SConscript.native

@@ -27,3 +27,4 @@ Import('*')
 
 env.GTest('args')
 env.GTest('call_type')
+env.GTest('command', 'command.test.cc', 'command.cc', 'args.cc')