Ignore improper_ctypes_definitions warning in test

```
warning: `extern` fn uses type `impl Future<Output = ()>`, which is not FFI-safe
  --> test_suite/tests/test.rs:77:70
   |
77 |     async unsafe extern "C" fn const_async_unsafe_extern_non_const() {}
   |                                                                      ^ not FFI-safe
   |
   = note: opaque types have no C equivalent
   = note: `#[warn(improper_ctypes_definitions)]` on by default

warning: `extern` fn uses type `impl Future<Output = ()>`, which is not FFI-safe
  --> test_suite/tests/test.rs:79:78
   |
79 |     pub async unsafe extern "C" fn const_async_unsafe_extern_non_const_pub() {}
   |                                                                              ^ not FFI-safe
   |
   = note: opaque types have no C equivalent

warning: `extern` fn uses type `impl Future<Output = ()>`, which is not FFI-safe
  --> test_suite/tests/test.rs:81:72
   |
81 |     const async unsafe extern "C" fn const_async_unsafe_extern_const() {}
   |                                                                        ^ not FFI-safe
   |
   = note: opaque types have no C equivalent

warning: `extern` fn uses type `impl Future<Output = ()>`, which is not FFI-safe
  --> test_suite/tests/test.rs:83:80
   |
83 |     pub const async unsafe extern "C" fn const_async_unsafe_extern_const_pub() {}
   |                                                                                ^ not FFI-safe
   |
   = note: opaque types have no C equivalent
```

test_suite/tests/test.rs

@@ -1,6 +1,6 @@
 #![cfg_attr(const_unstable, feature(const_extern_fn))]
 #![warn(rust_2018_idioms, single_use_lifetimes)]
-#![allow(clippy::missing_safety_doc, clippy::unused_async)] // this is test
+#![allow(clippy::missing_safety_doc, clippy::unused_async, improper_ctypes_definitions)] // this is test
 
 pub mod signature {
     #![allow(dead_code)]