test(nat): Have NAT use concurrency::thread, cleanup test organization

Signed-off-by: Manish Vachharajani <[email protected]>

Author: mvachhar

nat/Cargo.toml

@@ -5,6 +5,9 @@ edition = "2024"
 publish = false
 license = "Apache-2.0"
 
+[features]
+shuttle = ["concurrency/shuttle", "dep:shuttle"]
+
 [dependencies]
 ahash = { workspace = true }
 concurrency = { workspace = true, features = [] }