* test(fixtures): add the ability to set env variables in the scope of a single fixture

Define the custom_env function in your fixture to set environment variables.

* feat(server): add reading of tokens from file (#338)

Add support for reading a list of newline separated tokens from a file
which path is being supplied through the following environment
variables:

- AUTH_FILE
- DELETE_FILE

* docs(authentication): add new file option to readme (#338)

* test(fixtures): add auth and delete file tests and fix expiring file error

- add fixture for auth file testing
- add fixture for delete file texting
- expiring-file-upload would sometimes fail, so I increased the sleep
duration