Targeted API to forget a particular codec name.

Normally we don't care about the caching (as evidenced by the fact it took 14+ years to want this feature).

The intent with this API is to be able to selective purge a particular codec when we either care about getting the latest version of a specific codec (no current use cases) or to avoid the appearance of leaking references when looking up lots of custom codecs (this case).

I considered dropping the per-test naming scheme, but decided I liked the fact it kept these test instances orthogonal, even though they all use the shared TEST_CODECS registry. The lack of global side effects is also a benefit of just popping the named codec, rather than purging the entire cache.