Lean on FBXCTestExtendedCommands for obtaining shims
Summary:
This solves a few issues:
1) Allow for the commands implementation to cache intelligently and avoid over-fetching
2) Make passing the shims down easier since we can query the implementation
There are some other examples of places where the shims are used, but those can be changed in time to avoid needing to always inject from above
Reviewed By: jbardini
Differential Revision: D29458524
fbshipit-source-id: 40786a0f73a726cec1518b10d014ae4f78c84d57
