Unit test for t3c login cookie

[jpappa200]

This PR adds a unit test for the t3c cookie login to verify that if the user changes the t3c will not continue to use the same cookie for login.

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Which Traffic Control components are affected by this PR?

• Traffic Control Cache Config (t3c, formerly ORT)

What is the best way to verify this PR?

Run the t3c unit tests.

If this is a bugfix, which Traffic Control versions contained the bug?

PR submission checklist

• [x] This PR has tests
• [ ] This PR has documentation
• [ ] This PR has a CHANGELOG.md entry
• [x] This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)