Use perform_now in test inline! mode

[alachaum]

Originally raised in: #173

The test inline! mode is expected to run jobs immediately. The problem is that this immediate execution is implemented in the create task action on the backend (MemoryTask), which is invoked while enqueuing the job.

Since the job is executed while being enqueued, the server middlewares run inside the client middlewares. This means the client middleware may leave dangling locks that the server middlewares would normally clean, thus preventing successive jobs from running in the case of the unique_job extension.

Here is a typical example:

Cloudtasker::Testing.inline! do
  TestUniqueJobWorker.perform_async(1, 2)
  TestUniqueJobWorker.perform_async(1, 2)
end
# Actual: [[1, 2]]            (second job is rejected due to dangling locks)
# Expected:  [[1, 2], [1, 2]]

This PR addresses this issue by doing the following:

1. Ensure perform_now runs the client middlewares on top of the server middlewares. This ensures the full worker lifecycle is respected.
2. Stop implementing the inline! execution in the create action of the backend (MemoryTask)
3. Instead, rewire perform_async to perform_now when test inline! mode is enabled