LU-412 tests: Fix test script lookup in auster

LU-412 tests: Fix test script lookup in auster

When asked to run "lfsck", auster found "/usr/sbin/lfsck" instead of
"$LUSTRE/tests/lfsck.sh".  This patch restricts auster to look for test
scripts in only $LUSTRE/tests.

Change-Id: Iea521d06cdf1cea1a9bd80224f847275732d3447
Signed-off-by: Li Wei <liwei@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1078
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>