Set testing environment for CodeQL workflow

[henrymercer]

Previously we simply disabled uploading the status reports for all PR checks except ones that were there specifically to test the status report functionality. However when there's a lot of development activity on the CodeQL Action repo and the Actions runtime is being flaky, it's valuable being able to completely exclude CodeQL Action development from telemetry queries.

As drive-by improvements, this PR also makes TEST_MODE specific to the CodeQL Action and enables it in the check-codescanning-config Action. We also remove the INTERNAL_CODEQL_ACTION_DEBUG_LOC variable, which is redundant as of #1358.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[angelapwen]

Thanks for this, CODEQL_ACTION_TEST_MODE is much clearer to me 👍