mapping: fixed test case

The recently added cleanup of weak mappings during rehash makes
adding entries to a weak mapping semantically similar to running gc().
Using this analogy, the testcase before this commit assumed that

  mapping a = set_weak_flag(([]), 1);
  a[17] = ({ 17 });
  gc();

results in a having one entry, which is clearly false.

src/testsuite.in

@@ -6400,7 +6400,7 @@ test_do(add_constant("blatinka");)
     return !sizeof (a) && !sizeof (b);
   }]], 1);
   test_any([[{
-    mapping a = ([17:({17})]);
+    mapping a = ([17:17]);
     set_weak_flag (a, 1);
     for (int i = 0; i < 10; i++) a[class{}()] = i;
     mapping b = a + ([]);