WeakMap, WeakSet, WeakRef, and FinalizationRegistry all had tests
verifying what would happen if they were called with a value that wasn't
allowed as a weak value: before this proposal, that was a non-Object.
Now, allowed weak values are Objects, well-known Symbols, and unregistered
Symbols. That leaves registered Symbols that are still not allowed as weak
values.
This commit updates those tests to use a registered Symbol instead of an
unregistered Symbol; they should still pass, regardless of whether the
implementation has implemented symbols-as-weakmap-keys yet.
The tests are renamed as appropriate.
Also updates the frontmatter to the most current spec text, including the
CanBeHeldWeakly abstract operation.
See: #2850
Philip Chimento