Refs #36953 -- Moved non-backend-dependent BaseEmailBackendTests.

Relocated BaseEmailBackendTests that are _not_ dependent on the email
backend.

- In general, moved test cases to EmailMessageTests or SendMailTests
  as appropriate, and changed them to work with the testing outbox.

- Replaced BaseEmailBackendTests.test_send_verbose_name() with
  EmailMessageTests.test_unicode_display_name_in_from_email().
  (EmailMessageTests.test_address_header_handling() also partly covers
  the behavior, as well as Python's own message serialization tests.)

- Removed BaseEmailBackendTests.test_message_cc_header(), which was
  already covered by EmailMessageTests.test_cc*() (and Python's own
  message serialization tests).

- Replaced BaseEmailBackendTests.test_idn_send() with
  EmailMessageTests.test_idn_addresses() to cover from_email and cc.
  (EmailMessageTests.test_address_header_handling() already covered to.)

- Removed BaseEmailBackendTests.test_recipient_without_domain(), which
  was partly covered by EmailMessageTests.test_localpart_only_address().
  Updated the latter to cover a localpart-only from_email.

- Updated docstrings and comments to clarify a few tests that _do_
  depend on the email backend.

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