Added missing flags in run_build_test_target.py

Added --host flag for run-test and replaced test-art-gtest with
test-art-host-gtest to only run tests on host.

With a change in arg parsers, there is no need to put additional
--build-target flag.
For e.g.,
./test/testrunner/run_build_test_target.py  art-interpreter -j64
./test/testrunner/run_build_test_target.py  -j64 art-interpreter

Both of them will build and run target for art-interprerter.

Test: ./test/testrunner/run_build_test_target.py  art-interpreter -j64
      ./test/testrunner/run_build_test_target.py  -j64 art-interpreter

Change-Id: I8e7c108082319a9214905854edefd0abc8343f2a