test: Increment rend cache allocation before freeing

The rend_cache/entry_free was missing the rend cache allocation increment
before freeing the object.

Without it, it had an underflow bug:

  Sep 17 08:40:13.845 [warn] rend_cache_decrement_allocation(): Bug: Underflow
  in rend_cache_decrement_allocation (on Tor 0.4.5.0-alpha-dev
  7eef9ced)

Fixes #40125

Signed-off-by: David Goulet <dgoulet@torproject.org>