MPP-3852: Test verify_from_sns

[jwhitlock]

This PR adds tests for verify_from_sns, which verifies the signature in an SNS JSON body against the signing certificate. Previously, this code was untested, and mocked whenever called. This hides that OpenSSL.crypto.verify has been removed in 24.3.0 (see #5228). When running the tests, pytest will now report:

DeprecationWarning: verify() is deprecated. Use the equivalent APIs in cryptography.

My plan is to rebase PR #5228 after this merges, and we should then see test failures. I'll then replace pyopenssl and pem with the cryptography equivalents. The change will be the exception raised when verification fails.

[jwhitlock]

Here's the warning in CircleCI (build_test_backend):

=============================== warnings summary ===============================
emails/tests/sns_tests.py::test_verify_from_sns_notification_with_subject_ver1
emails/tests/sns_tests.py::test_verify_from_sns_notification_with_subject_ver1_fails
emails/tests/sns_tests.py::test_verify_from_sns_notification_without_subject_ver1
emails/tests/sns_tests.py::test_verify_from_sns_subscription_ver1
  /app/emails/sns.py:83: DeprecationWarning: verify() is deprecated. Use the equivalent APIs in cryptography.
    crypto.verify(

[groovecoder]

praise: very nice tests!