[BUGFIX] errorHandling setup in site based functional tests

Some functional tests using SiteBasedTestTrait create
a broken site config, example:

base: 'https://website.local/'
errorHandling:
  -
    -
      errorFluidTemplate: typo3/[...]/FluidError.html
      errorFluidTemplatesRootPath: ''
      errorFluidLayoutsRootPath: ''
      errorFluidPartialsRootPath: ''
      errorHandler: Fluid
      errorCode: 404

It should be:

base: 'https://website.local/'
errorHandling:
  -
    errorFluidTemplate: typo3/[...]/FluidError.html
    errorFluidTemplatesRootPath: ''
    errorFluidLayoutsRootPath: ''
    errorFluidPartialsRootPath: ''
    errorHandler: Fluid
    errorCode: 404

This is due to a superfluous array when creating the
site config. The patch adapts usages.

Change-Id: I953abb0cd5d3414e8ea259e73b47cc6b391b299a
Resolves: #95523
Releases: master
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71485


Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>