TestMD5Intrinsics and TestMD5MultiBlockIntrinsics don't test the intrinsics

Both TestMD5Intrinsics and TestMD5MultiBlockIntrinsics get the expected hash in the first iteration of computing MD5, and this iteration is run by the interpreter.

The tests later run some warm-up iterations and get the hash for comparison. The hash is supposed to be obtained from the intrinsics. However, WARMUP_ITERATIONS is set to 1 in DigestSanityTestBase, and the hash is not obtained from the intrinsics.

Reproduction:
1. Introduce a bug in MacroAssembler::fast_md5 (x86)
#define FF(r1, r2, r3, r4, k, s, t) \
  /*movl(rsi, r3); remove an instruction*/ \
  ...
2. TestMD5Intrinsics still passes.
3. Change WARMUP_ITERATIONS in DigestSanityTestBase to 600.
4. TestMD5Intrinsics fails.