folly/wangle: temporarily disable compilation of Thens.cpp

folly/wangle: temporarily disable compilation of Thens.cpp

Summary:
This code fails to compile with clang:dev, so don't try for now.
* folly/wangle/test/Thens.cpp: Don't attempt to compile test/Thens.cpp.
See 4412111 for details.  Prompted by clang:dev+MSAN effort, 4090011.

Test Plan:
Run this:
fbconfig --clang --with-project-version clang:dev -r folly/wangle
fbmake runtests
Failed before, passes with this patch.

Reviewed By: hans@fb.com

Subscribers: folly@lists, fugalh

FB internal diff: D1354751

Tasks: 4090011, 4412111