watchman: remove use of jansson private strbuffer type from bser test
watchman: remove use of jansson private strbuffer type from bser test

Summary:
this test pre-dates our use of C++.  We're now able to replace
strbuffer with std::string

Reviewed By: pkaush

Differential Revision: D17779597

fbshipit-source-id: cdcbc555ed3a2624df85c4388e961129d483b3c8

facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman