Testrunner: Fix --gdb issue

Testrunner was eating up the output of gdb.
With the change, it will print the entire run-test script output
with --gdb.

Test: ./art/test/testrunner/testrunner.py --gdb -t 001-H

Change-Id: I35254d0a67380946e1f033ec2a46a6bf272d4adc