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>

tests: Enable test running outside of gem5's source tree

The learning gem5 scripts currently assumes that the current working
directory is the root of gem5's source tree. This isn't necessarily
the case when running the tests using gem5's new test runner.

Change-Id: Ief569bbe77b1b3e2b0fb0e6c575fb0705bbba9b3
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>

tests: Update learning gem5 tests scripts with copyright

tests: Add tests for the Learning gem5 scripts

These tests will ensure that Learning gem5 scripts are always up to date with
the changes in the mainline of gem5.

Committed by: Nilay Vaish <nilay@cs.wisc.edu>