Running sentry tests no longer hits redis rate limit

[simzzz]

closes #357
After some research, I decided to go with the following changes:

• Use SELECT to switch the Redis DB for each test, making sure tests don't mess it up for one another
• Since tests still hit the rate limit even with SELECT I discovered that a common approach is running the tests in the database module sequentially. I used the serial_test crate for that.

[elpiel]

This will still have issues because of the auth tests.

Make sure to create a Pool struct for the tests that will automatically release the Database once the test is done with them.

it should also Flush the Database before releasing it to the pool

https://docs.rs/deadpool

[elpiel]

LGTM! Should be enough for our needs atm