Fix missing tokio features for tokio_async test

Author: Vincent Rouillé

foundationdb/Cargo.toml

@@ -54,4 +54,4 @@ num-bigint = { version = "0.3.0", optional = true }
 byteorder = "1.3.2"
 lazy_static = "1.4.0"
 log = "0.4.8"
-tokio = { version = "0.2.9", features = ["rt-core"] }
+tokio = { version = "0.2.9", features = ["rt-core", "rt-threaded", "macros"] }