test-webkitpy: executive_unittest fails if PYTHONPATH isn't set

test-webkitpy: executive_unittest fails if PYTHONPATH isn't set
https://bugs.webkit.org/show_bug.cgi?id=77058

Reviewed by Eric Seidel.

r105935 broke executive_unittest by changing which script we
were running and not ensuring that the PYTHONPATH was set up
properly.

This change also fixes a regression introduced recently where
test-webkitpy wasn't figuring out the path to Tools/Scripts
right, either (so running ./test-webkitpy would fail).

* Scripts/test-webkitpy:
* Scripts/webkitpy/common/system/executive_unittest.py:
(script_dir):
(ExecutiveTest.setUp):
(ExecutiveTest):
(ExecutiveTest.tearDown):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105954 268f45cc-cd09-0410-ab3c-d52691b4dbfc