Increasing failover time to avoid errors like

I0914 19:15:38.606657832 40 priority.cc:658] [priority_lb 0x611000009b40] child child2 (0x613001702bc0): failover timer fired, reporting TRANSIENT_FAILURE

Should prevent flakes in InitialUnavailable tests

Note: test had to be modified to reproduce more easily, will monitor the
test after this fix to ensure problem does not appear again.

test/cpp/end2end/xds_end2end_test.cc

@@ -4396,7 +4396,7 @@ class FailoverTest : public BasicTest {
  public:
   void SetUp() override {
     BasicTest::SetUp();
-    ResetStub(100, "");
+    ResetStub(500, "");
   }
 };