Fixes#3326
Originally I started out making this configurable, but realized it wasn't
worth the complexity. If you are restoring a snapshot, you really need to
wait until it's done, or something bad happens like losing leadership, which
will already trigger an error. Rather than have operators have to tune this
to cover whatever their snapshot size is, we just make it block here. There's
also already a bocking barrier right after the restore that hasn't been a
problem.
This creates a simplified helper for temporary directories and files.
All path names are prefixed with the name of the current test.
All files and directories are stored either in /tmp/consul-test
or /tmp if the former could not be created.
Using the system temp dir breaks some tests on macOS where the unix
socket path becomes too long.
