Fix flaky playground integration test

[ferranbt]

The playground integration test in CI is flaky. It fails around 1 out of 5 times.

[metachris]

Two example failed runs:

• https://github.com/flashbots/rbuilder/actions/runs/10357136291/job/28668659012
• https://github.com/flashbots/rbuilder/actions/runs/10356976852/job/28668219380

Error:

running 1 test
test integration::simple::tests::test_simple_example ... FAILED

failures:

---- integration::simple::tests::test_simple_example stdout ----
thread 'integration::simple::tests::test_simple_example' panicked at crates/rbuilder/src/integration/simple.rs:38:14:
called `Result::unwrap()` on an `Err` value: ProposalNotFound
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    integration::simple::tests::test_simple_example

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 72 filtered out; finished in 9.26s

[ferranbt]

I think this is fixed now in #160. I will reintroduce the test that was failing.