Figure out how to specify a datetime during attestation verification tests

[MasterKale]

There's been a long-standing TODO within attestation-specific unittests to figure out how to specify the time so that x5c certificate validation can succeed even after a certificate in the chain has expired. To date the tests patched the actual "verify cert chain" call to simply return True because otherwise most tests would fail with "certificate expired"-like errors. This obscured detection of other potential issues with the X.509 certificates, though, and so I've always wanted to find a better solution.

This issue rose to the top recently while addressing #243. I decided to take another look at potential solutions when I stumbled upon pyca/pyopenssl#735. This called out the set_time() method available on instances of pyOpenSSL's X509Store. After some trial and error, I settled on a pattern to enable specifying a datetime in the past, to a time between all cert's notBefore and notAfter values.

This PR adds a new webauthn.helpers.validate_certificate_chain._generate_new_cert_store method that can be patched using unittest.mock.patch to return a custom X509Store instance with a fixed datetime.datetime. Related TODOs in unittests have been updated to leverage this new-found capability and ultimately increase confidence in existing attestation tests.