Runs host_init_verifier on merged target files packages.

This verifies the init rc files in the merged result.

Bug: 163089173
Test: test_common.py
Test: Run merge_target_files.py to merge two target-files packages where
      one has init_rc errors. Observe script failure.
Test: Run merge_target_files.py on two good target-files packages,
      observe no failure.
Change-Id: I86c8e5a2bc07c2c1896ac40afd32bc1d055447ee