TeamCity: test utils refactor

[SarahFrench]

This PR:

• updates the test utils used in the tests for the TeamCity config code.
• changes error messages to be framed the correct way

Test utils:

Previously there was a getSubProject function that would retrieve a project that was nested 2 levels deep from a starting project. It was always given the root of the TeamCity project and was used to pull out projects from within the Google or Google Beta projects.

Now, getSubProject does something more logical given its name. You supply a Project and the name of a subproject within it, and it'll return a Project object for that subproject. This can be used to make assertions on, etc.

The original functionality of retrieving a project from 2 levels of nesting is now fulfilled by getNestedProjectFromRoot, which uses getSubProject internally.

This PR refactors existing acceptance tests to use the new test utils.

Error message framing

Messages needed to be rewritten to be pointing out why a given test would fail.

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[modular-magician]

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[SarahFrench]

I just rebased this branch to get the new stuff from main, and I'll update the new tests

[modular-magician]

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[SarahFrench]

Waiting to merge until after the service and project sweepers run today