fix(tests): fix bad assertion in mailer tests
fix(tests): fix bad assertion in mailer tests

https://github.com/mozilla/fxa-auth-server/pull/1737

r=rfk
eoger/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server