build: pass client flags to test programs

build: pass client flags to test programs

Fix build failures of the kind:

  CC       tests/bad-buffer-test.o
In file included from tests/weston-test-client-helper.h:28:0,
                 from tests/bad-buffer-test.c:28:
./protocol/wayland-test-client-protocol.h:35:28: fatal error: wayland-client.h: No such file or directory
compilation terminated.
make[1]: *** [tests/bad-buffer-test.o] Error 1

These are only triggerable if libwayland has not been installed
system-wide, but only in a custom prefix.

Since the Makefile already uses AM_CPPFLAGS, simply add
TEST_CLIENT_CFLAGS to test programs instead of dropping AM_CPPFLAGS.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>