tests: move libqtest.h back under qtest/

Since commit a2ce7dbd917 ("meson: convert tests/qtest to meson"),
libqtest.h is under libqos/ directory, while libqtest.c is still in
qtest/. Move back to its original location to avoid mixing with libqos/.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
diff --git a/tests/qtest/libqos/ahci.c b/tests/qtest/libqos/ahci.c
index 41c4be5..f53f12a 100644
--- a/tests/qtest/libqos/ahci.c
+++ b/tests/qtest/libqos/ahci.c
@@ -24,7 +24,7 @@
 
 #include "qemu/osdep.h"
 
-#include "libqtest.h"
+#include "../libqtest.h"
 #include "ahci.h"
 #include "pci-pc.h"