Added ClearSystemState in a unit test

There is another unit test that has been failing randomly on win32
build. Investigation has shown that the failure was caused by simd
register state is not reset appropriately in the fdct8x8 test. This
commit added ClearSystemState() in the teardown of this test, tests
showed it resolved the random failure issue for win32 build.

Related issue: https://code.google.com/p/webm/issues/detail?id=614

Change-Id: I9381d0c1a6f4b855ccaeef1aca8c417ac8c71ee2