Fix d8-os unit test to work with isolates.

Fix d8-os unit test to work with isolates.

We cannot use chdir to set the working directory on a per-isolate basis,
hence we need to specify absolute directories instead for this test to
work properly on multi-threaded runs.

R=yangguo@chromium.org
TEST=mjsunit/d8-os

Committed: https://code.google.com/p/v8/source/detail?r=10639

[Yang, 2012-02-08 10:58:10]

LGTM.