Fix cases where functions are called with a drive and no slash

Fix cases where functions are called with a drive and no slash

When a file is specified on a path that includes a drive letter
followed by a colon but no slash then it didn't always account
for the fact that this refers to the current path on that drive.
This fixes the problems in completeBaseName(), baseName() and
path().  Tests are also added for these three cases and some
others too.

Task-number: QTBUG-25353

Change-Id: I47a197c6af066f532442ad269be57597ec61303a
Reviewed-by: Irfan Omair <irfan.omair@gmail.com>
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>