QTimeZone: Fix isValidId()

Fix isValidId() which was failing valid IDs because it was splitting
name parts by \ instead of /.  it was also rejecting offset from UTC
formats names.  Add unit tests.

Task-number: QTBUG-35025
Change-Id: I4d23d2e54f4a9fac9afcc4eff0a02d6f4af21385
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>