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>

tests/main.py

@@ -10,7 +10,7 @@ from __future__ import print_function
 import sys
 import os
 
-base_dir = os.path.dirname(os.path.abspath(__name__))
+base_dir = os.path.dirname(os.path.abspath(__file__))
 ext_path = os.path.join(base_dir, os.pardir, 'ext')
 
 sys.path.insert(0, base_dir)