tests: Fix tests/main.py so it can be run from anywhere.

tests: Fix tests/main.py so it can be run from anywhere.

tests/main.py was trying to find paths relative to itself using the
string __name__ (which was __main__) when it should have been using the
string __file__ which holds the name of the file being executed.

Change-Id: I5ff4c42fc7d8b75ff6b96c3cde61baf731d84738
Reviewed-on: https://gem5-review.googlesource.com/c/15675
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>