crypto/tls: fix testHandshake close flakes

The flakes were introduced by me in CL 586655. It's unclear why only
FreeBSD seems affected, maybe other TCP stacks handle sending on a
half-closed connection differently, or aren't as quick to propagate the
RST over localhost.

Fixes #68155

Change-Id: I32a1b474a7d6531dbab93910c23568b867629e8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/602615
Reviewed-by: Cherry Mui <[email protected]>
Run-TryBot: Filippo Valsorda <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Roland Shoemaker <[email protected]>
Auto-Submit: Filippo Valsorda <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: FiloSottile

src/crypto/tls/handshake_test.go

@@ -491,9 +491,10 @@ func testHandshake(t *testing.T, clientConfig, serverConfig *Config) (serverStat
 		if got := string(buf); got != sentinel {
 			t.Errorf("read %q from TLS connection, but expected %q", got, sentinel)
 		}
-		if err := cli.Close(); err != nil {
-			t.Errorf("failed to call cli.Close: %v", err)
-		}
+		// We discard the error because after ReadAll returns the server must
+		// have already closed the connection. Sending data (the closeNotify
+		// alert) can cause a reset, that will make Close return an error.
+		cli.Close()
 	}()
 	server := Server(s, serverConfig)
 	err = server.Handshake()