util: Add unit tests for most remaining m5 utility commands.

The only two which still need unit tests are the more complex commands,
readfile and writefile.

Change-Id: Ib9984c71fb4449cbbbd1e2a43f3140975328d31f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27607
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/command/SConscript.native

@@ -26,6 +26,15 @@
 Import('*')
 
 command_tests = (
+    'addsymbol',
+    'checkpoint',
+    'dumpresetstats',
+    'dumpstats',
+    'exit',
+    'fail',
+    'initparam',
+    'loadsymbol',
+    'resetstats',
     'sum',
 )