Summary:
Change the folly/future/... tests to use mainly[*] the rvalue-qualified versions of get() and get(dur).

[*] Although the lvalue-qualified get is deprecated, it is not yet removed, so this diff leaves a test for the lvalue-qualified version. But the bulk of the tests are for the rvalue-qualified get.

Reviewed By: yfeldblum

Differential Revision: D8239792

fbshipit-source-id: c8a2e90cb5ab97a59ec97994c8c6a6ca27dab86e