increase writeLen for tests that want to trigger buffering

increase writeLen for tests that want to trigger buffering

Summary:
Freshly opened TCP sockets can have a send buffer size bigger
than wmem_default, because connection parameters are cached in the
route cache.  Some of the async socket tests assumed that writing 8M of
data was enough to guarantee blocking behavior, but since wmem_max is
20M on our systems the resulting tests are flaky.

Reviewed By: marcinpe

Differential Revision: D4292201

fbshipit-source-id: ba5d606d080330e455eee2b17bcae6cf546bf981