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

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

This feeds a fake file to the readfile command which is just a sequence
of incrementing 32 bit values. The incrementing values make sure that
the right region of the input file is being read at the right position,
and the relatively small size means there shouldn't be tons of zeroes
everywhere which can't be distinguished from each other.

Change-Id: I4286b1f92684f127c4885c29192c6c5244a61855
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27608
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>