Fix flaky netd_integration_test in Cuttlefish

Use vfork/posix_spawn  instead of fork.

Fix test items:
netd_integration_test
 TetherStartStopStatus
 ClatdStartStop

Bug: 124363517
Test: built, flashed, booted
      system/netd/tests/runtests.sh pass
      manual test with remove dnsmasq/clatd, netd works fine

Change-Id: I43880bd8693112fab46a0931457ff468ae01b305
2 files changed