Add a test for invalid alert types.

This doesn't hugely matter, but I noticed it was some missing coverage.

Change-Id: I3e425d47fbbeaacd9da2ae883f34e89b4562ec11
Reviewed-on: https://boringssl-review.googlesource.com/17184
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/test/runner/runner.go

diff --git a/ssl/test/runner/runner.go b/ssl/test/runner/runner.go
index 10e728e..f714c77 100644
--- a/ssl/test/runner/runner.go
+++ b/ssl/test/runner/runner.go
@@ -412,11 +412,14 @@
 	// which will be compared against the expected value.
 	testTLSUnique bool
 	// sendEmptyRecords is the number of consecutive empty records to send
-	// before and after the test message.
+	// before each test message.
 	sendEmptyRecords int
 	// sendWarningAlerts is the number of consecutive warning alerts to send
-	// before and after the test message.
+	// before each test message.
 	sendWarningAlerts int
+	// sendBogusAlertType, if true, causes a bogus alert of invalid type to
+	// be sent before each test message.
+	sendBogusAlertType bool
 	// sendKeyUpdates is the number of consecutive key updates to send
 	// before and after the test message.
 	sendKeyUpdates int
@@ -750,6 +753,10 @@
 			tlsConn.SendAlert(alertLevelWarning, alertUnexpectedMessage)
 		}
 
+		if test.sendBogusAlertType {
+			tlsConn.SendAlert(0x42, alertUnexpectedMessage)
+		}
+
 		testMessage := make([]byte, messageLen)
 		for i := range testMessage {
 			testMessage[i] = 0x42 ^ byte(j)
@@ -2097,6 +2104,21 @@
 			expectedError:     ":TOO_MANY_WARNING_ALERTS:",
 		},
 		{
+			name:               "SendBogusAlertType",
+			sendBogusAlertType: true,
+			shouldFail:         true,
+			expectedError:      ":UNKNOWN_ALERT_TYPE:",
+			expectedLocalError: "remote error: illegal parameter",
+		},
+		{
+			protocol:           dtls,
+			name:               "SendBogusAlertType-DTLS",
+			sendBogusAlertType: true,
+			shouldFail:         true,
+			expectedError:      ":UNKNOWN_ALERT_TYPE:",
+			expectedLocalError: "remote error: illegal parameter",
+		},
+		{
 			name: "TooManyKeyUpdates",
 			config: Config{
 				MaxVersion: VersionTLS13,