util: Add a "writefile" unit test to the m5 utility.

Change-Id: Ic0e8d5fbbd5b6d6b57f674cef6460f94206a5872
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27628
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>

util/m5/src/command/SConscript.native

@@ -37,6 +37,7 @@ command_tests = (
     'readfile',
     'resetstats',
     'sum',
+    'writefile',
 )
 
 Return('command_tests')