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/fuzz/qos_fuzz.c b/tests/qtest/fuzz/qos_fuzz.c
index dc891d8..c856d3d 100644
--- a/tests/qtest/fuzz/qos_fuzz.c
+++ b/tests/qtest/fuzz/qos_fuzz.c
@@ -22,7 +22,7 @@
 #include "exec/memory.h"
 #include "qemu/main-loop.h"
 
-#include "tests/qtest/libqos/libqtest.h"
+#include "tests/qtest/libqtest.h"
 #include "tests/qtest/libqos/malloc.h"
 #include "tests/qtest/libqos/qgraph.h"
 #include "tests/qtest/libqos/qgraph_internal.h"