test: check error message in test-http-outgoing-proto

PR-URL: #10943
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-http-outgoing-proto.js

@@ -7,7 +7,8 @@ const OutgoingMessage = http.OutgoingMessage;
 const ClientRequest = http.ClientRequest;
 const ServerResponse = http.ServerResponse;
 
-assert.throws(OutgoingMessage.prototype._implicitHeader);
+assert.throws(OutgoingMessage.prototype._implicitHeader,
+              /^Error: _implicitHeader\(\) method is not implemented$/);
 assert.strictEqual(
   typeof ClientRequest.prototype._implicitHeader, 'function');
 assert.strictEqual(