BUGFIX: Fix support for typo3fluid/fluid 2.15

[bwaidelich]

Adjusts tests that mocked AbstractViewHelper::renderChildren() that is no longer invoked with version 2.15

Fixes: #3389

[bwaidelich]

Note: This line is no longer required with Fluid 2.15 – but it allows the tests to pass with older versions of Fluid

[dlubitz]

Thank you

[lolli42]

Quick note.
You may be better off avoiding mocks in VH tests.
Both fluid standalone and typo3 core basically transitioned to "functional" tests: Feed a TemplateView with some fluid template test string that calls the VH, render(), assert result string.
This turns out to be much more easy to read and avoids internal knowledge within tests.
Bonus: It's trivial to do this twice to test if a VH behaves identical with "template not cached and template cached" (we found and fixed quite a few issues due to this).