Reduce runtime of slow test

[ngoldbaum]

On my main development machine - an M3 Macbook Pro - this test currently takes ~2 seconds, introducing a noticeable pause running cargo test --lib:

cargo test --lib "types::mappingproxy::tests::iter_mappingproxy_nosegv" -- -Zunstable-options --report-time
   Compiling pyo3 v0.23.5 (/Users/goldbaum/Documents/pyo3)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 2.39s
     Running unittests src/lib.rs (target/debug/deps/pyo3-246e609de32070b5)

running 1 test
test types::mappingproxy::tests::iter_mappingproxy_nosegv ... ok <2.079s>

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 717 filtered out; finished in 2.08s

After this PR, it drops down to 35 milliseconds:

running 1 test
test types::mappingproxy::tests::iter_mappingproxy_nosegv ... ok <0.035s>

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 717 filtered out; finished in 0.04s

The slowdown is much worse when I'm testing some thread safety-related changes under TSAN.

I don't think there's any particular reason this loop needs so many iterations but please let me know if you chose the parameters for this test for a reason I'm missing @KLMatlock!

[davidhewitt]

Yep I think this is fine to simplify. I think these tests originate from a long long time ago (maybe 2 prior iterations of PyO3's main object API or more) and were present to monitor for bugs related to getting borrowed reference counting wrong!