net: report timeouts more aggressively in Accept in the fake implementation

This ensures that if the listener has already timed out when Accept
is called, Accept always returns an error instead of instantaneously
accepting a connection that was already pending.

For #17948.

Change-Id: Iabef7121590df3dcc2fe428429d7c2bc2bcb6cd5
Reviewed-on: https://go-review.googlesource.com/c/go/+/557438
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/net/net_fake.go

@@ -325,14 +325,27 @@ func (ffd *fakeNetFD) accept(laddr Addr) (*netFD, error) {
 		incoming []*netFD
 		ok       bool
 	)
+	expired := ffd.readDeadline.Load().expired
 	select {
-	case <-ffd.readDeadline.Load().expired:
+	case <-expired:
 		return nil, os.ErrDeadlineExceeded
 	case incoming, ok = <-ffd.incoming:
 		if !ok {
 			return nil, ErrClosed
 		}
+		select {
+		case <-expired:
+			ffd.incoming <- incoming
+			return nil, os.ErrDeadlineExceeded
+		default:
+		}
 	case incoming, ok = <-ffd.incomingFull:
+		select {
+		case <-expired:
+			ffd.incomingFull <- incoming
+			return nil, os.ErrDeadlineExceeded
+		default:
+		}
 	}
 
 	peer := incoming[0]