tests: Use TEST_CLIENT for test client modules

We abused SIMPLE_CLIENT_LIBS before, but if you disable simple clients,
the test suite fails to link.  Use test client specific variables instead.

https://bugs.freedesktop.org/show_bug.cgi?id=71530

configure.ac

@@ -266,6 +266,8 @@ AC_SUBST(JPEG_LIBS)
 
 PKG_CHECK_MODULES(CAIRO, [cairo])
 
+PKG_CHECK_MODULES(TEST_CLIENT, [wayland-client])
+
 AC_ARG_ENABLE(simple-clients,
               AS_HELP_STRING([--disable-simple-clients],
                              [do not build the simple wl_shm clients]),,

tests/Makefile.am

@@ -94,7 +94,7 @@ weston_test_client_src =		\
 	wayland-test-protocol.c		\
 	wayland-test-client-protocol.h
 weston_test_client_libs =		\
-	$(SIMPLE_CLIENT_LIBS)		\
+	$(TEST_CLIENT_LIBS)		\
 	../shared/libshared.la		\
 	libshared-test.la