Mailer tests

[krskibin]

Related issues

closes #3463

Short description and why it's useful

Added unit tests for core/modules/mailer

Screenshots of visual changes before/after (if there are any)

Which environment this relates to

Check your case. In case of any doubts please read about Release Cycle

• Test version (https://test.storefrontcloud.io) - this is a new feature or improvement for Vue Storefront. I've created branch from develop branch and want to merge it back to develop
• RC version (https://next.storefrontcloud.io) - this is a stabilisation fix for Release Candidate of Vue Storefront. I've created branch from release branch and want to merge it back to release
• Stable version (https://demo.storefrontcloud.io) - this is an important fix for current stable version. I've created branch from hotfix or master branch and want to merge it back to hotfix

Upgrade Notes and Changelog

• No upgrade steps required (100% backward compatibility and no breaking changes)
• I've updated the Upgrade notes and Changelog on how to port existing VS sites with this new feature

IMPORTANT NOTICE - Remember to update CHANGELOG.md with description of your change

Contribution and currently important rules acceptance

• I read and followed contribution rules