[BUGFIX] Unit tests: Define TYPO3_REQUESTTYPE

Unit test boostrap does not initialize constants
TYPO3_REQUESTTYPE and TYPO3_REQUESTTYPE_*, but various
tests rely on this basic constants being properly defined
in core v7. With PHP 7.2, accessing undefined connstants
throws warnings.
Patch makes according bootstrap method public and calls
it during unit test bootstrap.

Change-Id: I450e440694385244c554adef00a9c74135d457f5
Resolves: #83211
Releases: 7.6
Reviewed-on: https://review.typo3.org/54925


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <no-reply@typo3.com>