Add test filter option to run_unit_tests

e.g. ./run_unit_test.sh net_test_hci.HciLayerTest.*

Useful since this script wraps the hard part of pushing the binary
to the device and then shell to running it.

This also allows multiple individual tests to be run, since the filter
is per-target rather than global.

Change-Id: I74a6511ab226fb55054cbb840bb47815e0fdc97e