Consider adding site setups to the testing framework

[benjaminkott]

Does your feature request relate to a problem?

When testing extensions, setting up the site to run simple functional tests
for view helpers that require some TypoScript to be loaded is more than a
challenge. I am frustrated because the core does not expose the SiteBasedTestTrait
to extension authors through the testing framework. It would ease the pain
and avoid copying this trait from extension to extension.

Use cases

• Simple integrated end-to-end testing components of your extension
• Simple functional tests of extension components that require a setup site to be present
  Example: ViewHelpers with defaults configured in TypoScript settings per site

Describe the solution you would like

As an extension author, I need a way, to create a site in a functional/-acceptance testing context
for all TYPO3 versions that require a site config TYPO3 v11/12 LTS.