Fix stubbing of SMIME certificate

We stub the outer method instead so that the value is not memoized for
succeeding specs

spec/lib/gitlab/email/hook/smime_signature_interceptor_spec.rb

@@ -36,7 +36,7 @@ RSpec.describe Gitlab::Email::Hook::SmimeSignatureInterceptor do
   end
 
   before do
-    allow(Gitlab::X509::Certificate).to receive_messages(from_files: certificate)
+    allow(Gitlab::Email::Hook::SmimeSignatureInterceptor).to receive(:certificate).and_return(certificate)
 
     Mail.register_interceptor(described_class)
     mail.deliver_now

spec/workers/emails_on_push_worker_spec.rb

@@ -157,7 +157,7 @@ RSpec.describe EmailsOnPushWorker, :mailer do
           end
 
           before do
-            allow(Gitlab::X509::Certificate).to receive_messages(from_files: certificate)
+            allow(Gitlab::Email::Hook::SmimeSignatureInterceptor).to receive(:certificate).and_return(certificate)
 
             Mail.register_interceptor(Gitlab::Email::Hook::SmimeSignatureInterceptor)
           end