Changes:
- util/sendmail now populates email files in the 'test-emails' directory.
    - util/sendmail does this in a serialized fashion based off of
      the test suite and name retrieved from PYTEST_CURRENT_TEST
      in the format: `<test_suite>_<test_function>.n.txt` where n
      is increased by one every time sendmail is run.
- pytest conftest fixtures have been added for test email setup;
  it wipes out old emails for the particular test function being run.
- New aurweb.testing.email.Email class allows developers to test
  against emails stored by util/sendmail. Simple pass the serial
  you want to test against, starting at serial = 1; e.g. Email(serial).

Signed-off-by: Kevin Morris <kevr@0cost.org>