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

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>