Improve test coverage for server_name extension.

Notably, this would have caught ed8270a55c3845abbc85dfeed358597fef059ea9
(although, apart from staring at code coverage, knowing to set resumeSession on
the server test isn't exactly obvious). Perhaps we should systematically set it
on all extension server tests; ClientHello extension parsing happens after
resumption has been determined and is often sensitive to it.

Change-Id: Ie83f294a26881a6a41969e9dbd102d0a93cb68b5
Reviewed-on: https://boringssl-review.googlesource.com/1750
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/test/runner/common.go b/ssl/test/runner/common.go
index f22f95a..9af3063 100644
--- a/ssl/test/runner/common.go
+++ b/ssl/test/runner/common.go
@@ -449,6 +449,10 @@
 	// SkipCipherVersionCheck causes the server to negotiate
 	// TLS 1.2 ciphers in earlier versions of TLS.
 	SkipCipherVersionCheck bool
+
+	// ExpectServerName, if not empty, is the hostname the client
+	// must specify in the server_name extension.
+	ExpectServerName string
 }
 
 func (c *Config) serverInit() {