Add integration tests for file get and put variations, especially stdin/stdout

[itamarst]

1. Unicode vs. bytes is one of the harder parts of the port.
2. Unit testing stdin/stdout interactions is likely to be quite fragile, given both quite different semantics on Python 2 and 3, and need to emulate things like stdin/stdout.

As such, integration tests of tahoe get and tahoe put with (a) binary files and (b) stdin/stdout usage seems like a good idea, even with nominally good unit test coverage.

[itamarst]

Python has a pipes module, but it does the stupid thing on Python 3 where it assumes unicode files. So going to do it manually.

[GitHub <noreply@…>]

In 05a1349/trunk:

Merge pull request #1066 from tahoe-lafs/3715-get-put-integration-tests

CLI get/put integration tests.

Fixes ticket:3715