Update E-mail tests to validate HTML

[ilmartyrk]

We have low coverage for e-mail tests. We should update the tests to validate html in final e-mail.

[anettlinno]

Triage 60. Est. dev. time 16 hours. Sending to development.

[BeccaMelhuish]

@ssin1901 does this still seem relevant to you? Will put to 'later' for now