[WebCrypto] Add WebCryptoAPI WPT for CryptoKey serialization

We added ML-DSA and ML-KEM with broken CryptoKey serialization, but
this wasn't exposed because there was no WPT test for it. Add a WPT test
for serialization by roundtripping a message with a CryptoKey and
comparing the exportKey results.

(The other main way to do CryptoKey serialization is to use an IndexedDB.
Posting messages was simpler.)

Bug: 512509718
Change-Id: I7499ec3a05b6b9e72f22c00e0815f4f847027fe3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7842729
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Hubert Chao <hchao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1631811}
NOKEYCHECK=True
GitOrigin-RevId: dc989b88b1c28d312d47d12bea298ffb76dba6d2