tests: Add Python unit tests for m5.util.convert

tests: Add Python unit tests for m5.util.convert

Python unit tests need to be run from within gem5. This change adds a
script to run unit tests (tests/run_pyunit.py) and a unit test for
m5.util.convert.

The tests can be run as follows:

  ./build/NULL/gem5.opt tests/run_pyunit.py

Change-Id: I80d1aabbe1d87b01b48280972f9418317e648779
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39377
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>