tests/libqtest: Use libqtest-single.h in tests that require global_qtest

Tests that require global_qtest or the related wrapper functions now
use the libqtest-single.h header that is dedicated for everything
related to global_qtest. The core libqtest.c and libqtest.h files are
now completely indepedent from global_qtest, so that the core library
is now not depending on a global state anymore.

Message-Id: <20190904130047.25808-7-thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>