init: don't generate tombstones for expected test failures

The firmware_handler.HandleAbort and subcontext.RecoverAfterAbort
tests intentionally abort in the child process to ensure that
ueventd/init can recover if their child processes die.  This generates
a tombstone which causes confusion.  This change resets SIGABRT to
SIG_DFL right before the abort(), so that the child processes will
exit normally without generating a tombstone or writing a crash to
logcat.

Bug: 169771958
Bug: 175383788
Test: run the above tests and verify no stack traces are printed to
      logcat and no tombstones are generated.
Change-Id: Ica09548d1c7a766bf5d9ff2e26c9fd558e85c7c1