Add regression test for issue 2415

Currently fails:

    error: unexpected end of input, unexpected token in nested attribute, expected ident
     --> test_suite/tests/regression/issue2415.rs:4:9
      |
    4 | #[serde()]
      |         ^

Author: dtolnay

test_suite/tests/regression/issue2415.rs

@@ -0,0 +1,5 @@
+use serde_derive::Serialize;
+
+#[derive(Serialize)]
+#[serde()]
+pub struct S;