Add option to persist temp local cache

[loosebazooka]

Makes it easier to inspect the client behavior on failures

[jku]

--repository-dump-dir provides a copy of the repository contents at client refresh time... I guess we could implement something similar for the client cache: After each refresh/download, make a copy of the client metadata cache.

Alternatively try to only do it for failing tests (in this case could likely just make a single copy of the cache after the test)... This might be technically more difficult though

[jku]

coming back to this: I think the easiest way to inspect client behaviour is still client debug output -- the test suite can really only do so much:

• the client should be invoked with enough verbosity
• pytest will print the client output on failure, but "-rA" will print it for all tests, see example in README

It feels like sensible the client output can easily provide any information that persisting local cache would -- and much more