libfs_avb: updating run_tests.sh

Adding `--host` for host-side test cases with atest, so we
still can run the test when there is no device connected.

Otherwise, TradeFed will emit the follow error:
  TradeFed subprocess exited early with exit code=7(NO_DEVICE_ALLOCATED).

Bug: None
Test: ./system/core/fs_mgr/libfs_avb/run_tests.sh
Change-Id: I0a593e6dc42861c2dc9cfff308cd7fd76b39ef2c