FileFeedStoragePreFeedOptionsTest fails if /tmp/foobar already exists with different ownership

[risicle]

Quick one - if, for instance, you're running the the scrapy test suite in some sort of CI setup where builds get performed by random different dedicated "build users", you'll end up in a situation where /tmp/foobar already exists, owned by a different user. At which point FileFeedStoragePreFeedOptionsTest will fail:

E       PermissionError: [Errno 13] Permission denied: '/tmp/foobar'

Fairly trivial to fix - you could put a random suffix on the filename.. better, generate a directory to contain it using tempfile.mkdtemp.. many options.

Full build failure log: https://nix-cache.s3.amazonaws.com/log/5fdywg4w7s6cyf224j42yn9619kw5gw1-python3.8-Scrapy-2.4.1.drv

Versions

Scrapy 2.4.1, python 3.8, macos 10.15