Add override keyword everywhere it's required

Clang catches some of these.  Add override keywords where we override
functions, per cpp core guidelines.

Change-Id: I66f9e6dff53e07f0057d86a806701593be16b188
Signed-off-by: Ed Tanous <edtanous@google.com>
diff --git a/test/async/context.cpp b/test/async/context.cpp
index 03480c9..7f821ed 100644
--- a/test/async/context.cpp
+++ b/test/async/context.cpp
@@ -4,7 +4,7 @@
 
 struct Context : public testing::Test
 {
-    ~Context() noexcept = default;
+    ~Context() noexcept override = default;
 
     void TearDown() override
     {