[BUGFIX] Failling unit test in GeneralUtilityTest for Windows system

In GeneralUtilityTest a test for the TYPO3_SITE_PATH parses the result
for a starting slash. This test fails on Windows system as the don't
provide slashes but drive letters. This patch skips the test on Windows
systems and provides a new test to check for absolute paths with drive
letters.

Releases: master, 6.2
Resolves: #64644
Change-Id: I87bb9c7df7debd6e5d44602330587eff3c1cf4fd
Reviewed-on: http://review.typo3.org/36437


Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>