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Add unit test for ThreadSafeStackContext (#71)

Ported from Uber's internal `tornado-extras` lib. Should've been added in #2.

From internal ticket T233805 by @sema:

> With the help of @recht, I've found a race (or unexpected semantics if you will) in Tornado's StackContext used by tornado-extras' RequestContext.
> 
> In short, StackContext more or less assumes it will only be expected on one thread. If you use it across threads (using wraps) then the context may be entered/exited in unexpected ways. This causes exceptions in RequestContext [like below], and other subtle errors.
> 
> ```
> AttributeError: 'RequestContextManager' object has no attribute '_prev_data'
> ```
> 
> I've included an explanation of the race(s) in the diff + a test case that triggers the error (revert the usage of ThreadSafeStackContext and the test should fail).

Signed-off-by: Yuri Shkuro <ys@uber.com>

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