crypto/tls: use default cipher suites in BenchmarkThroughput

CL 32871 updated the default cipher suites to use AES-GCM in
preference to ChaCha20-Poly1305 on platforms which have hardware
implementations of AES-GCM. This change makes BenchmarkThroughput
use the default cipher suites instead of the test cipher suites to
ensure that the recommended (fastest) algorithms are used.

Updates #17779.

Change-Id: Ib551223e4a00b5ea197d4d73748e1fdd8a47c32d
Reviewed-on: https://go-review.googlesource.com/32838
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

tls_test.go

@@ -668,6 +668,7 @@ func throughput(b *testing.B, totalBytes int64, dynamicRecordSizingDisabled bool
 				panic(fmt.Errorf("accept: %v", err))
 			}
 			serverConfig := testConfig.Clone()
+			serverConfig.CipherSuites = nil // the defaults may prefer faster ciphers
 			serverConfig.DynamicRecordSizingDisabled = dynamicRecordSizingDisabled
 			srv := Server(sconn, serverConfig)
 			if err := srv.Handshake(); err != nil {
@@ -681,6 +682,7 @@ func throughput(b *testing.B, totalBytes int64, dynamicRecordSizingDisabled bool
 
 	b.SetBytes(totalBytes)
 	clientConfig := testConfig.Clone()
+	clientConfig.CipherSuites = nil // the defaults may prefer faster ciphers
 	clientConfig.DynamicRecordSizingDisabled = dynamicRecordSizingDisabled
 
 	buf := make([]byte, bufsize)