Drop nightly dependency of compiletest

Author: dtolnay

test_suite/Cargo.toml

@@ -15,4 +15,4 @@ serde_derive = { path = "../serde_derive", features = ["deserialize_in_place"] }
 serde_test = { path = "../serde_test" }
 
 [dependencies]
-compiletest_rs = { version = "0.3", optional = true }
+compiletest_rs = { version = "0.3", optional = true, features = ["stable"] }