[TASK] Remove prophecy in EXT:core SiteTest

Besides the prophecy removal, also 3 assertions
are changed to assert concrete class instances
instead of generic interface. Thus avoid having
superflous tests and transform them into a good
test assertion.

Resolves: #98785
Releases: main
Change-Id: I7d0db4907106f7ef09bf8a5590afce4f57add644
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76713


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>