Clean up Director tests at Cache

[jhiemstrawisc]

There are a few ongoing issues with Director tests at the cache. First we noticed that unless the cache's high water mark is hit, Director tests aren't cleaned up (sometimes leading to hundreds of thousands of files in a single directory). This in turn starts affecting the filesystem, which bogs down the Cache.

We need two main fixes:

1. Director tests should be cleaned up periodically by the Pelican process so we're not relying on XRootD's purge process. This unfortunately means we need to deal with these files being owned by XRootD, but being modifiable by Pelican (a pitfall with dropprivs).
2. Director test files are already uniquely timestamped, but we need to nest them in a daily directory. That is, every day we should use a new directory to make sure that no directory contains more than a single day's worth of files.

[bbockelm]

Note that #3014 enables xrdhttp-pelican unconditionally for the cache. One nice thing about that is it introduces the evict API, meaning you can do this cleanup remotely from the director itself.