Migrates as many as possible test cases from Django mail module

[tkzt]

Totally, improved the test coverage from 63% to 94%.

fixes #25

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (06cd610) 63.07% compared to head (1d7741a) 93.67%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #64       +/-   ##
===========================================
+ Coverage   63.07%   93.67%   +30.59%     
===========================================
  Files           9        9               
  Lines         585      585               
===========================================
+ Hits          369      548      +179     
+ Misses        216       37      -179     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[greyli]

Could you move this commit to a separate PR? Let's review and merge that first.

[tkzt]

Could you move this commit to a separate PR? Let's review and merge that first.

Done~

[greyli]

Please resolve the conflicts and fix the lint issues:

  tests/test_backend.py:10:1: F401 'ssl.SSLError' imported but unused
  tests/test_mail.py:5:1: F401 'unittest.mock' imported but unused
  tests/test_mail.py:7:1: F401 'flask_mailman.utils.DNS_NAME' imported but unused

[tkzt]

Please resolve the conflicts and fix the lint issues:

  tests/test_backend.py:10:1: F401 'ssl.SSLError' imported but unused
  tests/test_mail.py:5:1: F401 'unittest.mock' imported but unused
  tests/test_mail.py:7:1: F401 'flask_mailman.utils.DNS_NAME' imported but unused

Finished that 🍻

[greyli]

Merged, thanks!