add fake connection to C API

crates/servicepoint_binding_c/src/connection.rs

@@ -3,7 +3,7 @@
 //! prefix `sp_connection_`
 
 use std::ffi::{c_char, CStr};
-use std::ptr::null_mut;
+use std::ptr::{null_mut, NonNull};
 
 use crate::{SPCommand, SPPacket};
 
@@ -46,6 +46,22 @@ pub unsafe extern "C" fn sp_connection_open(
     Box::into_raw(Box::new(SPConnection(connection)))
 }
 
+/// Creates a new instance of [SPConnection] for testing that does not actually send anything.
+///
+/// returns: a new instance. Will never return NULL.
+///
+/// # Safety
+///
+/// The caller has to make sure that:
+///
+/// - the returned instance is freed in some way, either by using a consuming function or
+///   by explicitly calling `sp_connection_free`.
+#[no_mangle]
+pub unsafe extern "C" fn sp_connection_fake() -> NonNull<SPConnection> {
+    let result = Box::new(SPConnection(servicepoint::Connection::Fake));
+    NonNull::from(Box::leak(result))
+}
+
 /// Sends a [SPPacket] to the display using the [SPConnection].
 ///
 /// The passed `packet` gets consumed.