tests/qtest: variable defined by g_autofree need to be initialized

tests/qtest: variable defined by g_autofree need to be initialized

According to the glib function requirements, we need initialise
 the variable. Otherwise there will be compilation warnings:

glib-autocleanups.h:28:3: warning: ‘full_name’ may be
used uninitialized in this function [-Wmaybe-uninitialized]
   28 |   g_free (*pp);
      |   ^~~~~~~~~~~~

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Havard Skinnemoen <hskinnemoen@google.com>
Message-Id: <20201118115646.2461726-2-kuhn.chenqun@huawei.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>