Add dynamic testing

In addition to requiring 'sidekiq/testing' and 'sidekiq/testing/inline',
a user can also call the following methods to control the test harness:

Sidekiq::Testing.fake!
Sidekiq::Testing.inline!
Sidekiq::Testing.disable!

Each of the above methods also accepts a block to execute within that
context before reverting to the state present before method invocation.

To query the current state, use the following methods:

Sidekiq::Testing.enabled?
Sidekiq::Testing.disabled?
Sidekiq::Testing.fake?
Sidekiq::Testing.inline?

Closes #1053