tests: Check for client/server-core.h inclusion

The purpose of wayland-*-protocol-core.h is to mimc the
wayland-*-protocol.h generated by scanner --include-core-only.
The only difference being what wayland-*-protocol.h should include.

Add an include check in the headers-protocol-core-test, to be sure that
a wayland-*-protocol.h generated with the --include-core-only option
properly includes wayland-*-core.h.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>