Add channel initializer closure to NIOAsyncTestingChannel.init (#3053)

Since nio 2.78, adding handlers to a pipeline requires the handlers to
be sendable.

That makes the
[NIOAsyncTestingChannel.init(handlers:loop:)](https://swiftpackageindex.com/apple/swift-nio/2.78.0/documentation/nioembedded/nioasynctestingchannel/init(handlers:loop:))
function cumbersome, because you cannot create handlers and then call
the function (unless your handlers are Sendable) even if you never use
the handlers elsewhere.

This PR adds a new initializer which takes a closure. The closure is run
on-loop before the channel is registered. This means we can do:

```swift
let channel = try await NIOAsyncTestingChannel {
    let handler = MyUnsendableHandler()
    try $0.pipeline.syncOperations.addHandler(handler)
}
```

Sources/NIOEmbedded/AsyncTestingChannel.swift

@@ -307,12 +307,27 @@ public final class NIOAsyncTestingChannel: Channel {
         handlers: [ChannelHandler & Sendable],
         loop: NIOAsyncTestingEventLoop = NIOAsyncTestingEventLoop()
     ) async {
-        self.init(loop: loop)
-
-        try! await self._pipeline.addHandlers(handlers)
+        try! await self.init(loop: loop) { channel in
+            try channel.pipeline.syncOperations.addHandlers(handlers)
+        }
+    }
 
-        // This will never throw...
-        try! await self.register()
+    /// Create a new instance.
+    ///
+    /// During creation it will automatically also register itself on the ``NIOAsyncTestingEventLoop``.
+    ///
+    /// - Parameters:
+    ///   - loop: The ``NIOAsyncTestingEventLoop`` to use.
+    ///   - channelInitializer: The initialization closure which will be run on the `EventLoop` before registration. This could be used to add handlers using `syncOperations`.
+    public convenience init(
+        loop: NIOAsyncTestingEventLoop = NIOAsyncTestingEventLoop(),
+        channelInitializer: @escaping @Sendable (NIOAsyncTestingChannel) throws -> Void
+    ) async throws {
+        self.init(loop: loop)
+        try await loop.submit {
+            try channelInitializer(self)
+        }.get()
+        try await self.register()
     }
 
     /// Asynchronously closes the ``NIOAsyncTestingChannel``.

Tests/NIOEmbeddedTests/AsyncTestingChannelTests.swift

@@ -65,6 +65,17 @@ class AsyncTestingChannelTests: XCTestCase {
         XCTAssertNoThrow(try channel.pipeline.removeHandler(name: "handler2").wait())
     }
 
+    func testClosureInit() async throws {
+        final class Handler: ChannelInboundHandler, Sendable {
+            typealias InboundIn = Never
+        }
+
+        let channel = try await NIOAsyncTestingChannel {
+            try $0.pipeline.syncOperations.addHandler(Handler())
+        }
+        XCTAssertNoThrow(try channel.pipeline.handler(type: Handler.self).wait())
+    }
+
     func testWaitForInboundWrite() async throws {
         let channel = NIOAsyncTestingChannel()
         let task = Task {