Use "QT += testlib" consistently

Use "QT += testlib" consistently

If a project uses "load(qttest_p4)" it doesn't need to add testlib to
the CONFIG or QT variables.  If a project does not use
"load(qttest_p4)", it should add "testlib" to the QT variable.

Change-Id: If28353713ccdfe13612682e3e88dadebe2f2eefd
Reviewed-on: http://codereview.qt.nokia.com/159
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>