OAuth2AuthenticationTokenMixinTests should anticipate a lower-precision timestamp

[jzheaux]

Instant can contain up to nine digits in its decimal part, but Jackson only supports microsecond precision by default. Given that, OAuth2AuthenticationTokenMixinTests should only assert that deserialized Instant values match their pre-serialized Instant values up to the microsecond.

Additional Details

This can be verified on Windows using JDK 11 and running OAuth2AuthenticationTokenMixinTests. Occassionally, the Instant obtained from the OS contains enough precision to cause rounding, making the assertions fail.