[TASK] Refactor PageRenderer tests

The existing tests of ext:core/Page/PageRenderer are in fact
functional tests since most of them trigger lots of dependencies
and call the main API method "render()" to check for something.

The patch moves most detail tests to two general functional tests
doing the main work and adds some more functional tests for more
specific cases.
The unit tests are stripped down to those parts that test only single
methods or smaller systems with proper mocking.

In effect, the full unit test suite does no longer depend on
initialization of $GLOBALS['LANG'], it is now removed from unit
test bootstrap.

Change-Id: I9f3f54ba52673e16bfcd303b46755e079d240de6
Resolves: #53558
Related: #51436
Related: #53476
Releases: 6.2
Reviewed-on: https://review.typo3.org/25304
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn