Remove reliance on util.inspect in snapshot tests

Summary:
According to Node's documentation: https://nodejs.org/api/util.html#util_util_inspect_object_showhidden_depth_colors

> The util.inspect() method returns a string representation of object that is intended for debugging. **The output of util.inspect may change at any time and should not be depended upon programmatically.**

Therefore, this diff switches over our RN Codegen snapshot tests to use a `JSON.stringify` call, followed by a replace of `"` with `'`. This gets the job done without compromising readability.

**Question:** Why do we not use `"`? A: Jest escapes all `"` in the snapshots, which makes reading the snapshots/using them in the console harder.

Changelog: [Internal]

Reviewed By: hramos

Differential Revision: D24157056

fbshipit-source-id: 2f1aa2df28ac3ed4aa17bcdbcd23846ddbf804cf