Add t3cutil for Go unit tests

[zrhoffman]

This PR fixes an issue encountered when running the unit tests from traffic_ops/app/bin/tests:

unit_golang_1  | github.com/apache/trafficcontrol/lib/varnishcfg imports
unit_golang_1  | 	github.com/apache/trafficcontrol/cache-config/t3cutil: no required module provides package github.com/apache/trafficcontrol/cache-config/t3cutil; to add it:
unit_golang_1  | 	go get github.com/apache/trafficcontrol/cache-config/t3cutil
atcn-tounit-1_unit_golang_1 exited with code 1

---

Which Traffic Control components are affected by this PR?

• Go unit tests

What is the best way to verify this PR?

Make sure the go unit tests pass when run from traffic_ops/app/bin/tests:

cd traffic_ops/app/bin/tests
docker-compose up --build

PR submission checklist

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

[kdamichie]

Fixes build. Approved