[tests] Start adding tests for hb-set.h

Fails now.  Fixing.
diff --git a/src/hb-set-private.hh b/src/hb-set-private.hh
index c736b69..8e03d68 100644
--- a/src/hb-set-private.hh
+++ b/src/hb-set-private.hh
@@ -137,6 +137,7 @@
 {
   hb_object_header_t header;
   ASSERT_POD ();
+  bool in_error;
 
   inline void init (void) {
     header.init ();
@@ -224,6 +225,11 @@
     for (unsigned int i = 0; i < ELTS; i++)
       elts[i] ^= other->elts[i];
   }
+  inline void invert (void)
+  {
+    for (unsigned int i = 0; i < ELTS; i++)
+      elts[i] = ~elts[i];
+  }
   inline bool next (hb_codepoint_t *codepoint) const
   {
     if (unlikely (*codepoint == SENTINEL)) {