Fix SSLContextTest to work in xplat
Fix SSLContextTest to work in xplat

Summary: Updated SSLContextTest.cpp to select the cert path based on `FOLLY_CERTS_PATH` preprocessor directive instead of hardcoded path

Reviewed By: aniketmathur

Differential Revision: D37855161

fbshipit-source-id: a8abfbe1b81b172c4e50b5a88690b21b035a848e

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly