Add missing virtual destructors to tests.

Change-Id: I916b1e646d9fe142d3483039eb6cc72464529832
diff --git a/test/cpu_speed_test.cc b/test/cpu_speed_test.cc
index 569ff26..be651b4 100644
--- a/test/cpu_speed_test.cc
+++ b/test/cpu_speed_test.cc
@@ -22,6 +22,7 @@
         libvpx_test::TestMode, int> {
  protected:
   CpuSpeedTest() : EncoderTest(GET_PARAM(0)) {}
+  virtual ~CpuSpeedTest() {}
 
   virtual void SetUp() {
     InitializeConfig();