Rust: flaky CI test

[jtdavis777]

See https://github.com/google/flatbuffers/actions/runs/22790589457/job/66116256431?pr=8778

Seems there is a flaky rust flexbuffers test. Rerunning the job lets it pass.

[RenzoMXD]

Problem: The flexbuffers_alloc_check test is flaky when run via cargo test. It uses a global allocation counter (static mut NUM_ALLOCS) to assert zero heap allocations on a warm builder reuse. The test harness's background threads occasionally make allocations that get counted, causing the assertion to fail intermittently (e.g., 193 vs 197).

Solution: Add test = false to both *_alloc_check binaries in Cargo.toml so cargo test skips them. They're already run separately via cargo run --bin in RustTest.sh, which has no test harness threads and passes reliably.

@jtdavis777 Please check my PR #8965

[jtdavis777]

Thanks @RenzoMXD !! I will look at this later today.