Add a Qt-API style QStandardPaths::setTestModeEnabled

QStandardPaths::enableTestMode has a verb in the imperative ("enable")
as the core word in the name. That indicates an action. The function
should not have had a parameter.

Instead, add a Qt-style setXXXEnabled function.

[ChangeLog][QtCore][QStandardPaths] QStandardPaths::enableTestMode is
deprecated and is replaced by QStandardPaths::setTestModeEnabled.

Change-Id: Ib26ad72d7c635890d2cb22ae9d44cbda08a6f17c
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: David Faure (KDE) <faure@kde.org>