add a UT for Wangle::Future for circular dependency

add a UT for Wangle::Future for circular dependency

Summary:
Making sure Wangle can handle circular dependencies.
There was an actual code issue in September that cause a crash in Atlas
Adserver due to this. The issue has been since fixed and Adserver's code
has been changed to avoid such circular dependency. We're adding a unit
test for this case.

Unit test is added as per our conversation on the Wangle group: https://www.facebook.com/groups/715931878455430/permalink/770180369697247/

Test Plan:
Add the same test on a code base from Sep 20th, observe that it fails:
https://phabricator.fb.com/P19352007

Run the unit test on latest code base - it succeeds

Reviewed By: hans@fb.com

Subscribers: atlas2-eng@, fugalh, folly-diffs@, leizhao, mchughj, kit, mpechuk

FB internal diff: D1761006

Tasks: 5384229

Signature: t1:1761006:1420238204:74ffb3fe8b88a25a23ade8e0990e69d405ea7f1e