variance_test: add missing ClearSystemState...

...to recently added SubpelVarianceTest

Change-Id: I8775e39fd5dbfba81ad42b79b47bf6dd6ca8cc0e
diff --git a/test/variance_test.cc b/test/variance_test.cc
index e65054f..207b6e7 100644
--- a/test/variance_test.cc
+++ b/test/variance_test.cc
@@ -218,6 +218,7 @@
     vpx_free(src_);
     delete[] ref_;
     vpx_free(sec_);
+    libvpx_test::ClearSystemState();
   }
 
  protected: