Make testInstantTCPConnectionResetThrowsError more reliable (#2810)

Tests/NIOPosixTests/SocketChannelTest.swift

@@ -611,7 +611,8 @@ public final class SocketChannelTest: XCTestCase {
         #if !os(Linux) && !os(Android)
         // This test checks that we correctly fail with an error rather than
         // asserting or silently ignoring if a client aborts the connection
-        // early with a RST during or immediately after accept().
+        // early with a RST before accept(). The behaviour is the same as closing the socket
+        // during the accept. But it is easier to test closing the socket before the accept, than during it.
         let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
         defer { XCTAssertNoThrow(try group.syncShutdownGracefully()) }
 
@@ -660,8 +661,13 @@ public final class SocketChannelTest: XCTestCase {
         XCTAssertNoThrow(try clientSocket.connect(to: serverChannel.localAddress!))
         XCTAssertNoThrow(try clientSocket.close())
 
-        // Trigger accept() in the server
-        serverChannel.read()
+        // We wait here to allow slow machines to close the socket
+        // We want to ensure the socket is closed before we trigger accept
+        // That will trigger the error that we want to test for
+        group.any().scheduleTask(in: .seconds(1)) {
+            // Trigger accept() in the server
+            serverChannel.read()
+        }
 
         // Wait for the server to have something
         XCTAssertThrowsError(try serverPromise.futureResult.wait()) { error in