tests: Add a helper to run external scripts

tests: Add a helper to run external scripts

Some tests are really just a wrapper around a test script in
configs/. Add a helper method to wrap these scripts to make sure they
are executed in a consistent environment. This wrapper sets up a
global environment that is identical to that created by main() when it
executes the script. Unlike the old wrappers, it updates the module
search path to make relative imports work correctly in Python 3.

Change-Id: Ie9f81ec4e2689aa8cf5ecb9fc8025d3534b5c9ca
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15976
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>