This adds tests to WeakMap, WeakSet, WeakRef, and FinalizationRegistry for
Symbols as weakly-held values. Regular symbols and well-known symbols are
both tested. These tests correspond to existing tests for Objects as
weakly-held values, but are put in separate files so that they can be
filtered out with the "symbols-as-weakmap-keys" feature flag.
Registered symbols are not allowed; this is already tested in the "cannot-
be-held-weakly" tests.
See: #2850