Followup to Bug #11752069 (former bug 43152)

Assertion `bitmap_is_set_all(&table->s->all_set)' failed in
handler::ha_reset

This followup fixes the compilation warning
  'test_bit' may be used uninitialized in this function
introduced by the previous patch.

unittest/mysys/bitmap-t.c

@@ -253,7 +253,7 @@ my_bool test_count_bits_set(MY_BITMAP *map, uint bitsize)
 
 my_bool test_get_first_bit(MY_BITMAP *map, uint bitsize)
 {
-  uint i, test_bit;
+  uint i, test_bit= 0;
   uint no_loops= bitsize > 128 ? 128 : bitsize;
 
   bitmap_set_all(map);