Fix tests that were broken and make importable with Go Modules

[leighmcculloch]

Summary

Fix up this package so that it can be imported with Go Modules. It is imported by stellar/go as a fork of github.com/throttled/throttled.

Goal and scope

The tests of this package no longer build because changes were made in c99eef3 that remove the store sub-package and related functionality. The tests weren't updated and still reference that deleted package. This has not caused stellar/go any problems because that repo is using Dep to manage dependencies, and Dep ignores all test files. stellar/go is moving to Modules (stellar/go#1634) which is stricter and 💥s when a module contains Go code that is dependent on packages that don't exist.

Summary of changes

• Remove deprecated_test.go as its counterpart deprecated.go was removed in c99eef3.
• Remove rate_test.go TestRateLimitUpdateFailures as it tests store functionality that was removed in c99eef3.
• Change rate_test.go TestRateLimit to use an injected Clock since the way it receives an injected time is through an injected store, and stores were removed in c99eef3.