[2d] (Subtask) Stress test S3 integration

[alaric-dotmesh]

As a dotscientist, I want Dotmesh's ability to pull from S3 into dots to work reliably, so that I can use them in my dotscience tasks.

Estimate for this issue is 2d, but is already included in epic's total.

ACs

• [x] An S3 stress test is written. It needn't be run by default in S3, but it should at least exist as a manual task.
• [x] It should pull at least one big file
• [x] It should confirm that everything gets pulled into Dotmesh correctly.
• [ ] It should confirm that no unnecessary disk storage is used.
• [ ] It should confirm that pulling from S3 happens in a reasonable timeframe.

[alaric-dotmesh]

Stress test bucket is now created as per the script; tests need elaborating to check more stuff (including local disk space usage and time).