hs-v2: The rend_cache/entry_free test underflows rend_cache_decrement_allocation()

For some reasons, the test rend_cache/entry_free recently started to fail with:

rend_cache/entry_free: 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)

And this is actually quite obvious by looking at the test that it would underflow all the time since the total cache size is never incremented but we decrement it when freeing.

This requires backport for the CI to pass on all our versions.

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