HKDF and testing progress in general

I've submitted pull requests 3183 (
https://github.com/w3c/web-platform-tests/pull/3183) and 3305 (
https://github.com/w3c/web-platform-tests/pull/3305) covering the sign,
verify, deriveBits, and deriveKey methods and welcome reviews. Earlier PRs
for generateKey, encrypt, decrypt, sign, verify, and digest have already
been merged.

PR 3305 has HKDF tests that are based on PR 16 of the spec (
https://github.com/w3c/webcrypto/pull/16). That PR is still open, and I
submitted a comment that I believe the behavior for key lengths that aren't
a multiple of 8 is unclear (and the other two deriveBits algorithms handle
it differently from each other). If the PR gets changed significantly I'll
have to revise the related tests, too.

I looked at wrapKey and unwrapKey, but since those include exporting and
importing keys, which Jim is working on tests for, I think it makes more
sense to build on Jim's tests when they are done than start a separate
effort now.

Once those are done, all the methods in the API will be covered.

Charlie