Add custom mvc setup method to testserver base class

[lars-erik]

Prerequisites

• I have added steps to test this contribution in the description below

Description

In order to use UmbracoTestServerTestBase with view files outside the test project, we need to be able to customize the IMvcBuilder setup. This addition opens up the setup so that the builder can be modified during setup.

For instance:

I would love for this to be backported to 13 as well.
(Here's to hoping all the bugs aren't found yet)

[github-actions]

Hi there @lars-erik, thank you for this contribution! 👍

While we wait for one of the Core Collaborators team to have a look at your work, we wanted to let you know about that we have a checklist for some of the things we will consider during review:

• It's clear what problem this is solving, there's a connected issue or a description of what the changes do and how to test them
• The automated tests all pass (see "Checks" tab on this PR)
• The level of security for this contribution is the same or improved
• The level of performance for this contribution is the same or improved
• Avoids creating breaking changes; note that behavioral changes might also be perceived as breaking
• If this is a new feature, Umbraco HQ provided guidance on the implementation beforehand
• 💡 The contribution looks original and the contributor is presumably allowed to share it

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[georgebid]

Hey @lars-erik, thank you for your PR 😄 someone on the core collaborators team will review this soon.

[georgebid]

Hey @lars-erik! I have checked your PR and I am happy to merge this in 🚀 Thanks again for the PR.