(folly) Account for different vsnprintf behavior on OSX in test

(folly) Account for different vsnprintf behavior on OSX in test

Summary: Apparently there is some dispute about the correct thing for `snprintf(buf, len, "bogus%", "foo")` to return. OSX says 5, Linux says -1. The code in `folly/String.cpp` depends on -1 to throw the exception this test was expecting.

Test Plan: make check on OSX

Reviewed By: chip@fb.com

Subscribers: exa, folly-diffs@

FB internal diff: D1770970

Tasks: 5936226

Signature: t1:1770970:1420680750:bafae2aa8327a52013e68af64a163323244ca03b