Add simple send integration tests

[mec]

We have out integration tests working well for the previews and it
follows that we can use them to test sending emails.

Whilst we cannot 'check' that Notify will do the right thing, we can
check that things appear to have gone well.

These tests use a real Notify account and the results of sending the
email will show up in the Notify console. No email is actually sent as
we use a test API key.

This work only includes the tests it does not run them in GitHub
actions, that will come next.

You can run them locally with a Notify API key:

docker build --build-arg RAILS_VERSION=5.2.8.1 . -t rails-5

docker run --rm -e "NOTIFY_API_KEY=$NOTIFY_API_KEY" rails-5:latest bin/rails test

[coveralls]

Pull Request Test Coverage Report for Build 8571274221

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.671%

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Totals
Change from base Build 8571103320:	0.0%
Covered Lines:	303
Relevant Lines:	304

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💛 - Coveralls