lavu: add missing log.h include in timer.h
fate: force the simple idct for xvid custom matrix test

The original test without a forced idct is still useful since it tests
the switching of the idct algorithm/permutation on x86 with MMX. MMXext
or SSE2. Make sure the test runs only if MMX inline asm is available and
force -cpuflags to all.
Add the required bitexact flag for both tests.