tests: clear system state after non-API calls

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

Change-Id: I668deb09aa7aa467709776e5819f936910698bc0

test/test.mk

diff --git a/test/test.mk b/test/test.mk
index fe5255d..806901d 100644
--- a/test/test.mk
+++ b/test/test.mk
@@ -1,3 +1,4 @@
+LIBVPX_TEST_SRCS-yes += clear_system_state.h
 LIBVPX_TEST_SRCS-yes += register_state_check.h
 LIBVPX_TEST_SRCS-yes += test.mk
 LIBVPX_TEST_SRCS-yes += acm_random.h