Add script to run host native unit tests

* Add run_host_unit_tests.py, which does
  1. Run "mmma -jN system/bt"
  2. Run all host native unit tests configured in HOST_TESTS
  3. Output structured XML GTest log output

Bug: 64687730
Test: ./test/run_host_unit_tests.py -j40
Change-Id: I389c67a2e0f60639d1bbfa20375364964b184233