Add a basic MTU test.

The minimum MTU (not consistently enforced) is just under 256, so it's
difficult to test everything, but this is a basic test. (E.g., without renego,
the only handshake message with encryption is Finished which fits in the MTU.)
It tests the server side because the Certificate message is large enough to
require fragmentation.

Change-Id: Ida11f1057cebae2b800ad13696f98bb3a7fbbc5e
Reviewed-on: https://boringssl-review.googlesource.com/2824
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/test/runner/common.go b/ssl/test/runner/common.go
index 4dd710c..443d464 100644
--- a/ssl/test/runner/common.go
+++ b/ssl/test/runner/common.go
@@ -590,6 +590,10 @@
 	// ExpectInitialRecordVersion, if non-zero, is the expected
 	// version of the records before the version is determined.
 	ExpectInitialRecordVersion uint16
+
+	// MaxPacketLength, if non-zero, is the maximum acceptable size for a
+	// packet.
+	MaxPacketLength int
 }
 
 func (c *Config) serverInit() {