Test suite fails if features/misc_files/* or parent directories have strict permissions

We share this directory (with libvirt FS sharing) and then do stuff like I copy the sample videos to "/home/amnesia/Persistent" as user "amnesia". If the Git repository was cloned or updated with a strict umask, then I see:

      Failed to copy file:

      cp: cannot open `/tmp/shared_video_dir/video.mp4' for reading: Permission denied
       (MiniTest::Assertion)
      ./features/step_definitions/common_steps.rb:755:in `/^I copy "([^"]+)" to "([^"]+)" as user "([^"]+)"$/'

I’ve also been told that the 9p FS sharing fails if features/misc_files itself isn’t readable by libvirt (possibly because of parent directories’ permission, by the way).

I think this requirement should at least be written down somewhere, and ideally the test suite would check if it’s satisfied before running stuff that will fail otherwise. I don’t think we can automatically “fix” those permissions without doing dangerous things like recursively making all parent directories world-readable.

This just hit me on isotester1.lizard, so I’m making this a subticket of #8539 (closed) => kytv, anonym: we need an assignee on it.

Feature Branch: test/8950-fs-shares-from-temp-dir

Parent Task: #8539 (closed)

Original created by @intrigeri on 8950 (Redmine)