I think this is a step closer to the answer above, since it does test for implicit templates.
# IMPORTANT!
# must copy https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/support/helpers/next_instance_of.rb
it 'renders foo_mail' do
allow_next_instance_of(described_class) do |mailer|
allow(mailer).to receive(:render_to_body).and_wrap_original do |m, options|
expect(options[:template]).to eq('foo_mail')
m.call(options)
end
end
body = subject.body.encoded
end