Fix flaky flexbuffers_alloc_check test in cargo test (#8965)

tests/rust_usage_test/Cargo.toml

diff --git a/tests/rust_usage_test/Cargo.toml b/tests/rust_usage_test/Cargo.toml
index 5b7152b..828224d 100644
--- a/tests/rust_usage_test/Cargo.toml
+++ b/tests/rust_usage_test/Cargo.toml
@@ -25,10 +25,12 @@
 [[bin]]
 name = "flatbuffers_alloc_check"
 path = "bin/flatbuffers_alloc_check.rs"
+test = false
 
 [[bin]]
 name = "flexbuffers_alloc_check"
 path = "bin/flexbuffers_alloc_check.rs"
+test = false
 
 [[bin]]
 name = "sample_flexbuffers"