Migrate off of archived firebase/OCMock SPM dependency

**Is your feature request related to a problem you're having? Please describe.**

The following dependency in the Package.swift has been archived for over a year by the firebase org:
```
.package(
name: "OCMock",
url: "https://github.com/firebase/ocmock.git",
.revision("7291762d3551c5c7e31c49cce40a0e391a52e889")),
```

**Describe the solution you'd like**

The firebase author forked this from the original repo which appears to fully support SPM now:
https://github.com/erikdoe/ocmock/blob/master/Package.swift

What work would be needed to make this dependency change in this Package to ease dependency resolution in projects that depend on GoogleSignIn?