[test] Fix test-subset on old distros (#758)

https://developer.gnome.org/glib/stable/glib-Testing.html#g-assert-cmpmem

test/api/test-subset.c

@@ -26,6 +26,10 @@
 
 #include "hb-test.h"
 
+#ifndef g_assert_cmpmem
+#define g_assert_cmpmem(m1, l1, m2, l2) g_assert_true (l1 == l2 && memcmp (m1, m2, l1) == 0)
+#endif
+
 /* Unit tests for hb-subset.h */
 
 static const char test_data[] = { 0, 0, 1, 0 };