tests: Specify the death test style to avoid flakiness.

As warned below (while running the test), the default death test style
(i.e. "fast") doesn't work well in a threaded context, which causes test
flakiness (timeout or early exit).

[WARNING] external/googletest/googletest/src/gtest-death-test.cc:836:: Death tests use fork(), which is unsafe particularly in a threaded context. For this test, Google Test detected 3 threads.

This CL specifies the death test styles to be "threadsafe" for the
following death tests.
- RangeSetTest.GetBlockNumber
- RangeSetTest.file_range
- ScreenRecoveryUITest.LoadAnimation_MissingAnimation

Test: mmma -j bootable/recovery
Test: Run recovery_unit_test on marlin. Test passes and the above
      warning is gone.
Change-Id: I245bbc09286702d5cb326f878c4391e842b66cc5

tests/unit/screen_ui_test.cpp

@@ -408,5 +408,7 @@ TEST_F(ScreenRecoveryUITest, LoadAnimation_MissingAnimation) {
   ASSERT_TRUE(ui_->Init(kTestLocale));
   TemporaryDir resource_dir;
   Paths::Get().set_resource_dir(resource_dir.path);
+
+  ::testing::FLAGS_gtest_death_test_style = "threadsafe";
   ASSERT_EXIT(ui_->RunLoadAnimation(), ::testing::KilledBySignal(SIGABRT), "");
 }