crypto/tls: updating test data fails

[titanous]

$ go test -update
...
Wrote testdata/Server-TLSv12-SNI
Wrote testdata/Server-TLSv12-SNI
Wrote testdata/Server-TLSv12-SNI
140735196164944:error:14094438:SSL routines:ssl3_read_bytes:tlsv1 alert internal error:s3_pkt.c:1461:SSL alert number 80
--- FAIL: TestHandshakeServerSNIGetCertificateError (0.01s)
    handshake_server_test.go:498: Error from Server.Write: Test error in GetCertificate
    handshake_server_test.go:572: Handshake failed
...
FAIL
exit status 1
FAIL    crypto/tls  1.978s

I've fixed this and will send a CL shortly.

/cc @agl

[titanous]

https://go-review.googlesource.com/8959