Test that we reject RSA ServerKeyExchange more thoroughly.

The old test just sent an empty ServerKeyExchange which is sufficient as we
reject the message early. But be more thorough and implement the actual
ephemeral key logic in the test server.

Change-Id: I016658762e4502c928c051e14d69eea67b5a495f
Reviewed-on: https://boringssl-review.googlesource.com/3650
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/test/runner/common.go b/ssl/test/runner/common.go
index b505c8f..ff8c2d7 100644
--- a/ssl/test/runner/common.go
+++ b/ssl/test/runner/common.go
@@ -562,9 +562,10 @@
 	// retransmit at the record layer.
 	SequenceNumberIncrement uint64
 
-	// RSAServerKeyExchange, if true, causes the server to send a
-	// ServerKeyExchange message in the plain RSA key exchange.
-	RSAServerKeyExchange bool
+	// RSAEphemeralKey, if true, causes the server to send a
+	// ServerKeyExchange message containing an ephemeral key (as in
+	// RSA_EXPORT) in the plain RSA key exchange.
+	RSAEphemeralKey bool
 
 	// SRTPMasterKeyIdentifer, if not empty, is the SRTP MKI value that the
 	// client offers when negotiating SRTP. MKI support is still missing so