Add unit tests to check codeQLDatabasesExperimental.configError value

[shati-patel]

Follow-up to #1866:

• Slight tweak to the ordering of when we set the context value: 8e6c155
• Adds two unit tests (one to check the case when the config is valid, and one when it's invalid): 9c9092f

Checklist

N/A - internal only! 🤶🏽

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.