tests: clear system state after non-API calls

add ClearSystemState() to reset MMX registers avoiding corrupting
subsequent tests.

Change-Id: I668deb09aa7aa467709776e5819f936910698bc0

test/sixtap_predict_test.cc

@@ -12,6 +12,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include "test/acm_random.h"
+#include "test/clear_system_state.h"
 #include "test/register_state_check.h"
 #include "test/util.h"
 #include "third_party/googletest/src/include/gtest/gtest.h"
@@ -48,6 +49,10 @@ class SixtapPredictTest : public PARAMS(int, int, sixtap_predict_fn_t) {
     dst_c_ = NULL;
   }
 
+  virtual void TearDown() {
+    libvpx_test::ClearSystemState();
+  }
+
  protected:
   // Make test arrays big enough for 16x16 functions. Six-tap filters
   // need 5 extra pixels outside of the macroblock.