testFencepostErrors() {
    const hasher = new Sha512();
    const A64 =
        'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa';
    const A128 = A64 + A64;
    for (let i = 110; i <= 113; i++) {
      hasher.update(A128, i);
      const digest = hasher.digest();
      const expected = crypt.hexToByteArray(TEST_FENCEPOST_VECTOR[i]);
      assertElementsEquals(`Fencepost ${i}`, expected, digest);
      hasher.reset();
    }
  }
Make sure that we correctly handle strings whose length is 110-113.
This is the area where we are likely to hit fencepost errors in the padding
code.