Add support for TYPO3 v10.0

[bnf]

Sorry, I didn't see #118 before, therefore this patchset has some duplications – still, I think this approach is a bit cleaner:

• uses class aliases for the testbase ViewHelper
• proper site configuration handling for frontend requests
• fixes site configuration to be read from (and caches to be written to) the temporary test instance directory, instead of the original root
• Does not promise ^10.0 compatibility, just ~10.0.0

[bnf]

Rebased on master to incorporate the recent removal of 7.6 support in https://github.com/Nimut/testing-framework/commit/a0737b49e32640022f8d004487df5ae0d69f4b28.

[IchHabRecht]

Resolved with #123