Fix getting of enviroment strings in testlib

Fix getting of enviroment strings in testlib

The standard C getenv() returns NULL if the requested environment
variable is not found.

In Qt4 and later, qgetenv() does not return a null pointer if the
requested environment string is not defined.  Instead it returns a
QByteArray containing an empty string.  If using qgetenv(), there is no
way to tell the difference between an undefined environment variable
and one which is defined to be the empty string.

In testlib, all calls to qgetenv() were checking whether the returned
QByteArray's constData() returned a null pointer, but that would never
happen.  These calls must instead check whether the QByteArray contains
a non-empty string.

Change-Id: I342f0e8b196896c26cccce3ff169fa1b9669b5ff
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>