qmake: require a drive in a DOS path for it to be absolute

For Q_OS_WIN, a path is only truly absolute if it includes a drive
letter; merely starting with a slash is not enough.  (We can't support
UNC paths, so don't even try: qmake runs various commands in the
source directory using CMD.exe, which doesn't support UNC as PWD.)
This requires, when resolving a path relative to a root, transcribing
the root's drive to such not-quite-absolute paths.

Changed QMakeGlobals, $$absolute_path() and $$relative_path() to now
use IoUtils::resolvePath() rather than delegating to QDir's absolute
path method, since that doesn't correctly recognize the need for a
drive letter (and qmake did run into problems with some paths, from
splitPathList and a failing test, as a result).

Moved existing ioUtils tests for handling of relative / absolute paths
out into separate functions and expanded significantly.  Fixed some
existing tests to use an absolute path where one is needed; added two
tests involving driveless (but rooted) paths; and fixed the test init
to set a value for QT_HOST_DATA/src property (the lack of which lead
to an assertion failure with this fix).

Task-number: QTBUG-50839
Change-Id: I2bfc13c1bfbe1ae09997274622ea55cb3de31b43
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
(cherry picked from qtbase/e86f3c018833141776db2d15772ba53995656eac)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>