react-native-github: set path mock to posix by default

Summary:
@public

Otherwise tests using `path` but not `fs` have an empty mock, as it happens on https://github.com/facebook/metro/issues/235#issuecomment-423722747

Reviewed By: mjesun

Differential Revision: D10008513

fbshipit-source-id: 673e2ca79c8105a68818985dc08fe7ccd5d13881

local-cli/__mocks__/path.js

@@ -17,4 +17,6 @@ function reset(platform) {
 
 mockPath.mock = {reset};
 
+reset('posix');
+
 module.exports = mockPath;