fix(sms): Use the real email sender when sending via MockNexmo

We were using a made up sender, which caused our email provider
to drop the messages on the ground when testing on fxa-ci. Messages
were reported as delivered, but never seen again.

lib/mock-nexmo.js

@@ -39,7 +39,8 @@ function MockNexmo(log, config) {
 
         // HACK: Enable remote tests to see what was sent
         mailer.sendMail({
-          from: `sms.${senderId}@restmail.net`,
+          from: config.smtp.sender,
+          sender: config.smtp.sender,
           to: `sms.${phoneNumber}@restmail.net`,
           subject: 'MockNexmo.message.sendSms',
           text: message