Add tests for DeviceRedirectionMITM

[xshill]

See #267.

Since the DeviceRedirectionMITM component needs changes anyway, this could be a good time for adding tests. Among the things that should be tested are:

• Duplicate files are deleted.
• Files are named after their hash (Use hash as name for intercepted files #261).
• All files get closed so we don't end up with too many open file descriptors.

[xshill]

After experimenting a bit with unittest.mock, it seems much simpler to do than I originally thought.

Here are some things we would like to test:

• Make sure that client credentials are logged
• Make sure that responses to forged requests don't get sent to the server
• Make sure that open files are properly closed when a close request is received
• Make sure that read responses only handles files that we've opened
• Make sure that duplicate files are deleted
• Make sure that the file mapping contains proper info (e.g: sha1 hash, remote path)
• Make sure that the file mapping gets saved when it's supposed to