types: Write test for header.Hash()

[ebuchman]

In #2670 we changes the header.Hash to use SimpleHashFromByteSlices instead of SimpleHashFromMap, since the former is a simpler and more straightforward design with less dependencies (eg. no need to build kvpairs or sort them).

We should write a test for the header.Hash. We can have both:

• a test using reflect to ensure the hash is computed with the fields in the right order
• a hardcoded test with actual values for the header and actual hash output that can serve as test vectors for others verifying header hashes

[ebuchman]

Note any hard-coded values will be impacted by the changes in #2713