Add test config to net_test_btcore

This change allows TradeFederation to run the test directly.
Refer to b/35882476 for design and discussion of this change.

Bug: 35882476
Test: local test
tradefed.sh run template/local --template:map test=net_test_btcore

Change-Id: I336af9f5360540cec5fb406d38c90706891af5be