CI: Add platform update testing

[kammce]

Testing Procedure

1. Setup: run sammy platform install
2. Capture the current commit hash to save for later.
3. sammy platform update and expect it to return that there is nothing to do. Check that the commit hash is the same as before.
4. sammy platform update but this time, move the master head back 2 commits and save the commit hash. Ensure that the update results in a difference between the previous has and the current hash, and that the current has is equal to the saved hash in part 2.