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

init/firmware_handler_test.cpp

@@ -79,6 +79,8 @@ TEST(firmware_handler, HandleAbort) {
 }
 
 int HandleAbort(int argc, char** argv) {
+    // Since this is an expected failure, disable debuggerd to not generate a tombstone.
+    signal(SIGABRT, SIG_DFL);
     abort();
     return 0;
 }

init/subcontext_test.cpp

@@ -202,6 +202,8 @@ BuiltinFunctionMap BuildTestFunctionMap() {
 
     // For RecoverAfterAbort
     auto do_cause_log_fatal = [](const BuiltinArguments& args) -> Result<void> {
+        // Since this is an expected failure, disable debuggerd to not generate a tombstone.
+        signal(SIGABRT, SIG_DFL);
         return Error() << std::string(4097, 'f');
     };
     auto do_generate_sane_error = [](const BuiltinArguments& args) -> Result<void> {