test: Move qtests to a separate directory

The tests directory itself is pretty overcrowded, and it's hard to
see which test belongs to which test subsystem (unit, qtest, ...).
Let's move the qtests to a separate folder for more clarity.

Message-Id: <20191218103059.11729-6-thuth@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>