Fix failing tests

The following tests were marked with @pytest.mark.skipif(True, reason='Need to investigate') since they were failing in the n17 branch when we setup CircleCI. Each one should be investigated and fixed if possible.

• inbox/test/api/test_invalid_account.py::test_search_endpoints
• inbox/test/api/test_invalid_account.py::test_write_endpoints
• inbox/test/api/test_messages.py::test_message_label_updates
• inbox/test/api/test_threads.py::test_thread_label_updates
• inbox/test/api/test_validation.py::test_account_validation
• inbox/test/api/test_validation.py::test_valid_email
• inbox/test/auth/test_generic_auth.py::test_update_account
• inbox/test/auth/test_ssl_auth.py::test_imap_connection
• inbox/test/auth/test_ssl_auth.py::test_smtp_connection
• inbox/test/auth/test_ssl_auth.py::test_auth
• inbox/test/imap/test_smtp.py::test_use_plain
• inbox/test/scheduling/test_syncback_logic.py::test_actions_are_claimed
• inbox/test/scheduling/test_syncback_logic.py::test_actions_claimed_by_a_single_service

Start by running the listed pytest tests and inspecting their @pytest.mark.skipif markers, beginning with the files under inbox/test/api and inbox/test/auth. Compare the failures with the n17 CircleCI setup and investigate each skipped case. Done means the tests are fixed where possible, the skip markers are removed, and all listed tests pass.