dependencydiff unit tests interact with GitHub API

[spencerschrock]

Describe the bug
Something in the dependency diff unit test makes a single REST call.
https://github.com/ossf/scorecard/blob/846fb19724435f0c6465af44adf505ad6b2fc941/dependencydiff/dependencydiff_test.go

This causes unrelated PRs (such as #2704) to occasionally fail their unit-test step as the unauthenticated rate limit for API calls is low enough (60/hr for an IP address) that we occasionally hit it.

Reproduction steps
Steps to reproduce the behavior:

1. Check how much of your current API quota you've used:

curl -H "Accept: application/vnd.github+json" \
-H "Authorization: Bearer $GITHUB_AUTH_TOKEN" \
https://api.github.com/rate_limit

2. Run the dependecy diff unit tests (either make unit-test or individually)
3. Check your used API quota again, it should have gone up by one

Expected behavior
A unit test doesn't consume API quota. The relevant test should be modified to remove the call, or only run when e2e tests are run.