Improve test coverage in parts of Minder with low coverage.

[dmjb]

Certain parts of Minder have low unit test coverage. In at least one recent incident, the incident could have been prevented if there had been some unit test coverage of the codepath in question (there was none).

Based on some recent pain points, and looking at stats from Coveralls, some of the files we should focus attention our attention on are:

1. internal/providers/github/common.go
2. internal/controlplane/handlers_profile.go
3. internal/controlplane/handlers_evalstatus.go
4. internal/verifier/sigstore/container/container.go
5. internal/util/helpers.go
6. internal/controlplane/handlers_projects.go
7. internal/controlplane/handlers_user.go
8. internal/controlplane/handlers_oauth.go
9. ...tions/alert/security_advisory/security_advisory.go
10. internal/controlplane/handlers_artifacts.go