Fix cd(), cdUp() QDir member functions docs regarding nonreadable dirs

The documentation implies that cd() and cdUp() functions return false if
the new directory is not readable, but that is not the case. It is an
obvious mistake in the documentation, because cd'ing into a nonreadable
directory is perfectly valid.

Provided also with a test to verify that cd() actually returns true with
nonreadable directories.

Change-Id: I4aa3e859b35c64266df510a203574e3701aea77c
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>