No tombstones under normal bionic unit tests run.

Change a test to use a signal that does not trigger a tombstone.

Also changed the gwp_asan_integration.DISABLED_assert_gwp_asan_enabled
test to inherit from the SilentDeathTest to prevent tombstones
being generated.

Test: Ran tests and no tombstones generated.
Change-Id: I0f104704829dde692aa515a63dea1c6971278c29
diff --git a/tests/stdio_test.cpp b/tests/stdio_test.cpp
index b2195e7..b0f59bb 100644
--- a/tests/stdio_test.cpp
+++ b/tests/stdio_test.cpp
@@ -1049,11 +1049,13 @@
 }
 
 TEST(STDIO_TEST, popen_return_value_signal) {
-  FILE* fp = popen("kill -7 $$", "r");
+  // Use a realtime signal to avoid creating a tombstone when running.
+  std::string cmd = android::base::StringPrintf("kill -%d $$", SIGRTMIN);
+  FILE* fp = popen(cmd.c_str(), "r");
   ASSERT_TRUE(fp != nullptr);
   int status = pclose(fp);
   EXPECT_TRUE(WIFSIGNALED(status));
-  EXPECT_EQ(7, WTERMSIG(status));
+  EXPECT_EQ(SIGRTMIN, WTERMSIG(status));
 }
 
 TEST(STDIO_TEST, getc) {