The directory service caches certain directory entries, or has them set by
other callers using `.set()`. When we try to override those directories with
custom providers, the cached value still takes precedence.
It might make more sense to just store the directory entry values directly in
the directory service's hash, but this patch just takes the less obtrusive
path of clearing cached values for keys that we override.
This patch also fixes the few instances where add-on manager tests leave files
in the global temporary directory which are now caught by the shutdown
assertions.
MozReview-Commit-ID: Jq92TngLO1L
62f14a5089