chore(dependencies): enable futures-util/alloc for tests

Cargo.toml

@@ -43,7 +43,7 @@ want = { version = "0.3", optional = true }
 [dev-dependencies]
 form_urlencoded = "1"
 futures-channel = { version = "0.3", features = ["sink"] }
-futures-util = { version = "0.3", default-features = false, features = ["sink"] }
+futures-util = { version = "0.3", default-features = false, features = ["alloc", "sink"] }
 http-body-util = "0.1"
 pretty_env_logger = "0.5"
 spmc = "0.3"
@@ -85,7 +85,7 @@ client = ["dep:want", "dep:pin-project-lite", "dep:smallvec"]
 server = ["dep:httpdate", "dep:pin-project-lite", "dep:smallvec"]
 
 # C-API support (currently unstable (no semver))
-ffi = ["dep:libc", "dep:http-body-util"]
+ffi = ["dep:libc", "dep:http-body-util", "futures-util?/alloc"]
 
 # Utilize tracing (currently unstable)
 tracing = ["dep:tracing"]