RequestContext: fix segfaults in the tests
RequestContext: fix segfaults in the tests

Summary:
Update all of the RequestContext tests to use a test fixture that helps ensure
they all start from a consistent state.

Otherwise tests can end up running with RequestContxt settings left over from
previous test functions, which can affect their results.

In particular, several tests leave behind data for the "test" key.  This
causes subsequent calls to `setContextData()` to not actually set new data for
this key, but to instead reset its value to null.  This would cause many tests
to segfault since they did not check if `getContextData()` returned null
before dereferencing the result.

Reviewed By: yfeldblum, A5he

Differential Revision: D9233617

fbshipit-source-id: ce7a7c738592305f01b16fec7796505ea192ede4

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly