tests: move libqtest.h back under qtest/

Since commit a2ce7dbd ("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>