resolver_test: Subscribe to death notification of service IDnsResolver

Currently, unit test may pass as even if Netd crashes and restarts quickly.
Subscribe the service death recipient of service IDnsResolver which is
loaded by Netd to detect any Netd death all the time. It helps to early
detect the bug likes b/123044370 before submitting commit.

Test: as follows
      1. built, flashed, booted
         system/netd/tests/runtests.sh passes
      2. kill netd manually during running resolv_integration_test and
         check if the death recipient aborts testing.

Change-Id: I8e5dd045e23aec4f7454172ce24f6860e1288f35