Async test return values aren't Sendable, so tests fail to compile

[colinmarc]

In the async tests, several of the tests return structs that aren't declared Sendable:

swift-bridge/crates/swift-integration-tests/src/async_function.rs

Line 17 in 225380e

async fn rust_async_return_struct() -> AsyncRustFnReturnStruct;

This is an error in swift 6:

/home/colinmarc/dev/swift-bridge/integration-tests/Sources/SharedLib/Generated/SharedLib.swift:4135:26: error: sending 'rustFnRetVal' risks causing data races
4133 |     return await withCheckedContinuation({ (continuation: CheckedContinuation<RustString, Never>) in
4134 |         let callback = { rustFnRetVal in
4135 |             continuation.resume(with: rustFnRetVal)
     |                          |- error: sending 'rustFnRetVal' risks causing data races
     |                          `- note: task-isolated 'rustFnRetVal' is passed as a 'sending' parameter; Uses in callee may race with later task-isolated uses
4136 |         }
4137 |

See also #269, #288, #280, #310.